The secretary of the IWUSP was the Austrian Friedrich Adler of the SPÖ; other prominent members were Otto Bauer and Julius Martov.
It published Nachrichten der Internationalen Arbeitsgemeinschaft Sozialistischer Parteien ("News of the IWUSP").
Poale Zion (labour Zionist) leaders David Ben-Gurion and Shlomo Kaplansky were active in the movement behind the Two and a Half International.
The IWUSP criticized the other two Internationals for what it perceived to be dogmatism, and advocated that more consideration should be given to the particularities of the political situation in each country.
In the 1930s, a similar effort was made to create an international between the reformism of the Second and the Leninism of the Third, as the London Bureau of left-wing socialist parties.