Asynchronous computer mediated communication (CMC) would seem to be an ideal medium for supporting development in student argumentation. This paper investigates this assumption through two studies. The first study compared asynchronous CMC with face-to-face discussions. The transactional and strategic level of the argumentation (i.e. measures of argumentation quality) observed in face-to-face discussions was higher than that observed in CMC discussions; however, this difference was not reflected in the students' subsequent level of argumentation. The second study compared a CMC discussion consisting of students from the same degree programme with those from different degree programmes. Students from the same degree programme participated more in the discussion than students from different degree programmes, but the structural quality of the argumentation was higher for students from different degree programmes. The implications of these findings for the methods used to assess the quality of argumentation are discussed. (Contains 3 notes.)