The World Federation of Scientific Workers (WFScW) and UNESCO share roots in the Social Relations of Science (SRS) movements and in the Franco-British scientific relations which developed in the 1930s. In this historical context (the Great Depression, the rise of Fascism and the Nazi use of science, the social and intellectual fascination for the USSR), a new model of scientific internationalism emerged, where science and politics mixed. Many progressive scientists were involved in the war efforts against Nazism, and tried (...) to prolong their international commitments into peacetime. They contributed to the establishment of the WFScW and of UNESCO in 1945–1946. Neither the WFScW nor UNESCO succeeded in achieving their initial aims. Another world emerged from the immediate post-war years, but it was not the world fancied by the progressive scientists from the mould of scientific internationalism. The aim of this article is to follow the path from the Franco-British networks towards the establishment of the WFScW and UNESCO; from an ideological scientific internationalism towards practical projects. It is to understand how these two bodies came to embody two different scientific internationalisms during the Cold War. (shrink)