This paper examines the diplomatic role of Romeu de Marimon, a key figure in the Crown of Aragon’s relations with Mamluk Egypt during the reign of James II (1291-1327). By analysing archival sources and Marimon’s family network background, this study highlights the overlapping of political, commercial, and religious spheres in late medieval Mediterranean diplomacy. The study pays particular attention to the interplay between royal authority and local institutions, and to the ambiguous role of excommunication in the practice of diplomacy.