Written environments in online learning enable professional discourse to be analysed in depth and provide greater knowledge for improving learning and for planning and delivering courses aimed at professional development. Until now, research into professional discourse has highlighted the importance of interaction in the development of professional discourse, the need to analyse professional discourse as much from a historical point of view as from a situational one and the influence the demands of the context have on this discourse. This study takes this triple result and delves further into the professional discourse verbalised by 21 educational psychologists taking an online practice development course, and incorporates an integrated historical and situational analysis. The professional discourse reflected in interaction observed in a debate activity and in the study material for the course was analysed to discover the principal sources of power and sources of content. The historical analysis revealed four sources of power: researchers of recognised standing, the author of the study material, the expert professional acting as teacher and the primary sources of information. The situational analysis revealed three sources of content: the tasks and contexts, the theoretical models, and the approach and professional practice intervention models. Sources of power, sources of content and also the type of request made by the teacher are elements identified in the discourse that take on a special relevance in the design and planning of courses for professionals in interactive learning environments. (Contains 1 figure, 1 footnote, and 5 tables.)