The rise of sociology as a relatively autonomous area of knowledge at the turn of the 20th century coincides with the heyday of neo-Kantian philosophies at both French and German-speaking universities.  For each context, scholars have identified various exchanges between neo-Kantian thought and some of the founding figures of sociology (e.g. Weber, Simmel, Durkheim). While the parallel has been noticed, no systematic comparison and analysis of cross-national transfers has been presented. The pertinence of the prima facie similarity thus remains obscure (chapter 1). My dissertation project sets out to fill this gap. It draws on Bourdieusian field theory, the comparative history of disciplinary organisation (Lepenies 1985, Ringer 1992) and recent proposals for a transnational history of the social sciences (Heilbron/Guilhot/Jeanpierre 2008). The resulting approach seeks to explain differences and similarities in the use of neo-Kantian topoi in sociology in terms of the structure of the respective academic fields and corresponding dispositions of the agents. The aim of the first part of the thesis is to provide elements for an historical evaluation of the interrelations of neo-Kantian philosophies and early sociologies in French and German-speaking academic fields (chapter 2). The second part then focuses specifically on one crucial aspect of the exchange between neo-Kantianism and sociology, a process described as a “sociologization” of the Kantian a priori. In both French and German-speaking contexts, the Kantian categories and the forms of sensibility, space and time, were reinterpreted as socially constituted and historically variable cognitive structures. Programmes in the sociology of knowledge were developed to investigate them empirically (chapter 3). The project illuminates successively (1) the genesis, (2) the reception and (3) the diffusion of these sociological theories of the a priori in a transnational perspective. It highlights the role of neo-Kantian theories of knowledge in the genesis of sociologies of knowledge (chapter 4); analyzes reactions to the associated challenges of relativism, reflexivity and “sociologism” in three national contexts (chapters 5 and 6); and offers elements for understanding the further trajectories and the systematic prospects of sociological theories of the a priori (chapter 7 and 8). The project thus contributes not only to a fuller picture of the relations between philosophy and sociology in France and the German-speaking world at the turn of the 20th century. It also illuminates the genealogy of notions of social categories, social space and social time which have become ubiquitous in today’s social and cultural sciences.