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Mixed Commissions, Mixed Blessing: On the British-Portuguese Anti–Slave Trade Treaty of 1817

Randall Lesaffer

In 1817 and 1818, the British government scored a significant gain in its fight for the abolition of the international slave trade. In the span of a year, three European powers, the Netherlands, Portugal, and Spain agreed to a treaty which restricted the slave trade on the Atlantic and imposed an enforcement mechanism. The first of these treaties was that of London of 28 July 1817 with Portugal (67 CTS 373).

Even since the British parliament had adopted the Act for the Abolition of the Slave Trade (47 Geo. 3, c. 36 (Eng.)) in 1807, His Majesty’s Government had taken the battle for abolition to the slave-trading nations of Europe and America and used the British Navy to help stop it. During the Napoleonic War, the navy had been able to operate under the laws of war and neutrality to stop and search foreign merchant ships. The approaching peace had mobilized the abolition movement and its supporters in the British government to try and commit the great powers of Europe to a ban of the slave trade at the Congress of Vienna. This had resulted in the Declaration of the Eight Courts Relative to the Universal Abolition of the Slave Trade of 8 February 1815 (63 CTS 473), which made its way into the Final Act of the Congress of 9 June 1815 (64 CTS 453). The Declaration contained a strong moral condemnation of the slave trade and a commitment of its signatories to abolish it, but it stipulated neither a legal obligation to do this effectively nor a clear timeline within which effective measures had to be taken.

In the years following the Congress of Vienna, Britain stepped up its efforts for abolition and relaunched its diplomatic offensive, which it had already started before Vienna, towards slave-trading nations to commit themselves to bilateral agreements. Among these countries, Portugal was a major target. The Portuguese possessions on the west coast of Africa held important slave-trading centres while the plantation economy in Brazil was a prime demander.

Portugal had been a key ally of Britain in the war against Napoleonic France. It had been from here that Arthur Wellesley (1769–1852), the Duke of Wellington, had launched his counterattack against the French occupation of Spain. In the Alliance Treaty of Rio de Janeiro of 19 February 1810 (61 CTS 41), the Prince Regent of Portugal, who was then residing in Brazil, had promised to prohibit his subjects to participate in the slave trade in any territories on the African coast which did not belong to Portugal and where the slave trade had been stopped (Article 10).

Subsequently, at Vienna, even before the Declaration of the Eight Courts had been signed, the British delegates had made a bilateral treaty with Portugal ‘for the prevention of the slave trade’. In the Treaty of 22 January 1815 (63 CTS 459), the Portuguese crown committed itself to a partial abolition of the slave trade. Article 1 of the treaty stipulated that the Portuguese government would prohibit its subjects from buying slaves or participating in the trade in Africa north of the equator. The logic of this was that it excluded direct traffic through the shortest route between the Portuguese possessions in the Angola region and the plantations in the greater area of Rio de Janeiro. From its side, Britain conceded that its navy would thenceforth do nothing to prevent the Portuguese slave trade in the southern Atlantic (Article 2). While the 1815 treaty made the traffic in slaves north of the equator and the Americas illegal for the Portuguese, it acknowledged the lawfulness of the traffic south of the equator towards Portuguese colonies. It did not provide any enforcement mechanism.

The achievement of the 1817 treaty was that it introduced an enforcement regime, which gave Britain and its navy a right to actively intervene against Portuguese and Brazilian slave traders. The regime rested on three pillars. First, Article 3 of the treaty imposed on the Portuguese crown the duty which would make participation in unlawful slave trade a punishable offence within two months after the ratification of the treaty.

Second, both states granted one another’s navies the right to stop and search merchant ships which were suspected to be engaged in unlawful slave trade, and to detain them (Article 5). The treaty did not extend the abolition of the slave trade to the traffic between the Portuguese possessions in Africa south of the equator and Brazil. This remained lawful (Articles 1 and 2).

Third, the signatories decided to create two mixed commissions, one in each power’s territory, which would have jurisdiction over captured ships (Article 8). The British court would be established at Freetown, Sierra Leone, and the Portuguese one in Rio de Janeiro. Each power would appoint a ‘Commissary Judge’ and a commissioner of arbitration (Regulation for the Mixed Commissions, attached to the Treaty of 28 July 1817, Article 2). In case the judges could not reach agreement, one of the arbitrators would be selected by lot to settle the case. The mixed commissions were given final adjudicating power, without the possibility of higher appeal, to rule over the lawfulness of the capture of a ship. In case it decided that the ship had been involved with unlawful traffic in slaves, the ships and its cargo would be confiscated and the slaves on board would be restored to freedom.

In the months after having signed the treaty with Portugal, Britain concluded similar treaties with Spain (Treaty of Madrid, 23 September 1817, 68 CTS 45) and the Kingdom of the Netherlands (Treaty of The Hague, 4 May 1818, 68 CTS 403). This led to the creation of two more mixed commissions in Havana, Cuba, and in Suriname.

Over the decades of their operation, these and other mixed courts founded for similar purposes made decisions with regards over 600 captured ships, the vast majority at Freetown. In total, between 1819 and 1846, some 80,000 slaves were freed and received their certificates of emancipation. For many of these, this was not the end of their misery as many would be unable to make their way home and would become primary targets for recapture.

The treaties of 1817–1818 which led to the mixed commissions certainly constituted a step forward in the fight for abolition, but they did not entail an unequivocal rejection of slavery, or even of the slave trade. The British-Portuguese treaties of 1815 and 1817 actually confirmed the lawfulness of the slave trade between the Portuguese colonies south of the equator. Moreover, the rules and regulations that governed the workings of the mixed commissions took great care to protect proprietary rights over slaves as well as ships and the procedural defences of the slave traffickers. The mixed commissions worked under the logic of traditional prize courts, like those in admiralty, which had to decide whether the capture of a ship was lawful or not. For the mixed commissions, this meant that before they could declare a capture lawful and release the slaves on board, a double test had to be met. First, it had to be proven that the ship had engaged in unlawful slave trade, which under the Portuguese treaty was a restricted category. Second, the ship had to have been captured according to the procedural instructions to the navies that were appended to the treaty. In the case of the Portuguese treaty, a major procedural restriction was that the ship had to be captured north of the equator. A ship involved in unlawful trade was thus not necessarily forfeit.

The 1817 treaty between Britain and Portugal did add an important instrument for abolition through the mixed commissions. But it was found unsatisfactory from the start and its gaps became a subject of almost constant political contention in Westminster. It was only in 1842, long after Brazil had gained its independence, that a new, far more restrictive treaty was made between London and Lisbon (Treaty of Lisbon, 3 July 1842, 93 CTS 255). In the years afterward, the Portuguese crown finally started to make a serious effort towards the effective prosecution of slave traders. This development led to a dramatic decline in slave traffic, proving to be much more effective than the mixed commissions had been and making those commissions largely redundant.


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Leslie Bethell, ‘The Mixed Commissions for the Suppression of the Transatlantic Slave Trade in the Nineteenth Century’, Journal of African History, 7 (1966) 73–93.

Emily Haslam, ‘International Criminal Law and Legal Memories of Abolition: Intervention, Mixed Commission Courts and “Emancipation”’, Journal of the History of International Law, 18 (2016) 420–47.

Jenny S. Martinez, The Slave Trade and the Origins of International Human Rights Law (Oxford: Oxford University Press, 2012).

Farida Shaikh, ‘Judicial Diplomacy: British Officials and the Mixed Commission Courts’, in Keith Hamilton and Patrick Salmon (eds.), Slavery, Diplomacy and Empire: Britain and the Suppression of the Slave Trade, 1807–1975 (Eastbourne: Sussex Academic Press, 2012) 42–64.