This study investigates whether combining chat discussion and construction of an argument diagram stimulates students to formulate new ideas in practising argumentation. In this study, 16 secondary school students discussed vivisection and gender equality in pairs using both free and structured chat tools. In structured chat, the students selected and completed partial sentences provided by the computer. After the discussion, they jointly constructed either argument diagrams freely based on the previous discussions with an Internet tool or modified a diagram the computer had constructed automatically during the structured chat. The freely constructed diagrams contained more of the students' prior knowledge than the modified diagrams. However, the different types of diagrams did not differ significantly in breadth, depth, or balance of argumentation. Thus, free construction of argument diagrams seems to activate students to incorporate their prior knowledge into those diagrams.