Background. Research has shown the importance of careful teacher support during collaborative group work to promote productive discourse between students (Webb, 2009). However, this research has traditionally focused on face-to-face communication. The role of online teacher guidance of small-group computer-mediated discussions has received little attention, especially in secondary school classroom settings. Researchers of computer-supported collaborative learning (CSCL), on the other hand, have traditionally focused on software-embedded features, such as scripts, to support a-synchronous peer dialogue, and less so on human guidance of synchronous group discussions. Aims. The main aim of the present in vivo, experimental study is to examine whether online teacher guidance can improve the quality of small-group synchronous discussions, and whether different types of guidance (epistemic or interaction guidance) affect these discussions differently, when compared to an unguided condition. The second goal of this study is to explore potential differences between all-female and all-male discussion groups. Sample. Eighty-two 9th graders (three classrooms) and six teachers from a rural high school in Israel. Results. Whereas epistemic guidance only improved aspects of the argumentative quality of the discussion, interaction guidance only improved aspects of collaboration. Discussions of all-girls groups scored higher on aspects of collaboration and argumentative quality, compared to all-boys groups. Conclusions. The findings show that teacher guidance of synchronous, online discussions in classrooms is realizable and reasonably reaches its intended goals. Training should be focused on acquiring various guidance strategies to augment their beneficial effects. Furthermore, future research should pay more attention to potential gender differences in peer-to-peer argumentation.