Cutting across the common distinction between learning to argue and arguing to learn, this research is concerned with arguing to learn argumentative knowledge, or broadening and deepening understanding of the space of debate. A secondary school experiment compared broadening/deepening of understanding of the space of a debate on genetically modified organisms, using either a CHAT tool, or else an argument-diagram tool (DREW: Dialogical Reasoning Educational Web tool) that was designed as a medium for interactive debate. Although there was no significant difference between the quality of students' texts before and after debating across the conditions, a new interaction analysis method ("Rainbow") revealed differences in students' expression and elaboration of arguments.