Joint construction of new knowledge demands that persons can express their statements in a convincing way and explore other people's arguments constructively. For this reason, more knowledge on different means to support collaborative argumentation is needed. This study clarifies whether structured interaction supports students' critical and elaborative argumentation. The study compares the quality of secondary school students' argumentation during structured and unstructured chat interaction. The data consist of 16 dyadic chat discussions: 8 discussions concerned vivisection and 8 gender equality. Half of the discussions were carried out through structured chat, and the other half through unstructured chat. The results suggest that a structured chat environment evokes counterargumentation, also in topics that do not spontaneously provoke conflicting viewpoints. Further, structured chat seems to equalize communication between females and males. Overall, the results indicate that the further investigation and design of pedagogical means to structure collaborative argumentation is a worthwhile enterprise. (Contains 11 tables.)