Abstract This article examines Anglo-Portuguese relations in the middle of the nineteenth century, particularly conflicts over territorial claims in West and East Africa. It examines how these conflicts were de-escalated and why they did not tear asunder the long-standing, if asymmetrical, alliance between Britain and Portugal. After briefly surveying Anglo-Portuguese relations in the early modern period and in the first half of the nineteenth century, the article focuses on the way that conflicts were resolved through third-party arbitration between the 1850s and 1870s. Drawing on archival research in Portugal and Britain, the article contributes to the rich historiographies on informal empire, the partition of Africa, and the emergence of international law in the context of imperial conflict and collaboration.