We designed an e-learning model to promote critical thinking about basic science topics in online communities of students during work placements in higher education. To determine the effectiveness and efficiency of the model we explored the online discussions in two case studies. We evaluated the quantity of the interactions by looking at quantitative data of the discussion "threads" and we evaluated the quality of the discussion by content analysis of the individual messages. Both the procedural facilitation of the discussion and the instrument for content analysis were based on Garrison's "Practical Inquiry model of Cognitive Presence". Furthermore, we explored the experiences of the students and moderators by interviewing them and we organised their perceptions using the framework of an activity system. On the basis of the quantitative and qualitative data we conclude that the e-learning model was successful in establishing a dialogue among a group of students and an expert during work placements at different locations. The "Practical Inquiry model" was useful in facilitating a sustained on-topic discourse involving critical thinking. Although the amount of critical thinking was moderate, the results suggest ways to increase integration and resolution activities in the online discussions. (Contains 2 tables and 7 figures.)