In this article two studies on the use of diagrams in computer-supported collaborative learning are compared. Focus is on the way argumentative diagrams can be used during collaborative learning tasks, more specifically how diagrams support argumentative interaction between students when they discuss ill-defined topics. The main goal is to discover how diagram construction before discussion, and diagram construction during discussion, influence the way students explore the space of debate during discussion. Twenty pairs of 16/17-year-old students were randomly selected from 126 pairs. Ten pairs worked with a diagram before discussion and ten during discussion. The research showed that students use diagrams in very different ways, ranging from a means for talking to just a notebook. Our expectation that using a diagram during discussion leads to more depth in discussion than using one before discussion, was not confirmed. Possible explanations for this finding are structure of the task, and the way students interpreted the goal of the task.