International scientific institutions are preferred settings to explore prominent topics in the transnational approaches to the recent history of science. These include: the dichotomy between universalism and contextualism in the historical evolution of exact sciences; the historical transformation of the ideals of scientific internationalism; the tensions between these historically changing ideals and scientists’ national allegiances; the balance between scientific and diplomatic activities of such institutions; and the issue of how the complex ideological, socio-political roles these institutions play affected the actual evolution of science.

Inspired by these overreaching themes, the paper presents a conceptual framework for pursuing a comparative analysis of IAU and IUPAP. A bird’s-eye view of the parallel historical developments of these two unions shows that, in spite of their common origins in the highly politicized context of the post-WWI reconstruction of international cooperation, IAU and IUPAP followed diverging trajectories in many respects. It is argued that a deeper study of these differences might give important insights to properly understand the relevance of specific disciplines’ scientific needs in the way the ideals and practices of scientific internationalism were actualized through the 20th century.