peer reviewed ; This article deals with a phenomenon which appears in a particular context: the institutionalisation of international law between 1869 (creation of the Revue de droit international et de législation comparée) and 1873 (birth of the Institut de Droit International), through the international networks of the Belgian lawyer Gustave Rolin-Jaequemyns. He was close to Pasquale Mancini, John Westlake, James Lorimer, Franz Von Holtzendorff, but also to the least-known Michael Corr-Van der Maeren, a publicist. Until this period of crystallisation this field was not structured and represented an aggregate of individualities. What can we say about such a process of institutionalisation on the basis of unpublished archives? The history of international law, until today, especially studied by lawyers, was very neglected by historians, whose methods and sources are complementary to those of the lawyer. Our contribution wishes to contribute to the development of a history of international law written by historians.