Purpose The article discusses findings from a research project on the communication history of the League of Nations. It departs from the League's normative goal of “open diplomacy”, which, from an analytical standpoint, can be framed as an “epistemic project” in the sense of a non-linear and ambivalent negotiation by communication of what “open diplomacy” should and could be. The notion of the “epistemic project” serves as an analytical concept to understand this negotiation of open diplomacy across co-evolving actors' constellations from journalism, PR and diplomacy. Design/methodology/approach The study employs a mixed-method approach, including hermeneutic document analysis of UN archival sources and collective biography/prosopography of 799 individual journalists and information officers. Findings It finds that the League's conceptualisations of the public sphere and open diplomacy were fluent and ambivalent. They developed in the interplay of diverse actors' collectives in Geneva. The involved roles of information officers, journalists and diplomats were permeable, heterogenous and – not least from a normative perspective – conflictive. Originality/value The subject remains under-researched, especially from the perspective of communication studies. The study is the first to approach it with the described research framework.