Women researchers in postdoc career stages still do not participate to the same extent in international academic mobility as men do. As a novel approach to better understand gender inequalities in international academic mobility, we investigate the structure of research collaborations and bring into focus the host researchers. Our central theoretical tool is the distinction between compositional and behavioral homophily. Using quantitative data from two German Humboldt Foundation programs sponsoring collaborations between international researchers and academic hosts in Germany, we ask (a) to what extent are gender-homophilous research collaborations caused by compositional or behavioral homophily; (b) what factors influence gender-based homophily; and (c) whether men or women tend more towards gender-homophilous collaborations. We detected behavioral homophily in research collaborations mainly when taking the potential pool of eligible women researchers as the reference group. The results on influencing factors are ambiguous and need further investigation. Furthermore, our results show that men tended more towards gender-homophilous collaborations than women. Our results suggest that gender inequalities in international academic mobility are linked to behavioral homophily and that exploring the role of host researchers is crucial to understanding—and overcoming—the under-representation of women in international academic mobility.