The main aim of this study is to promote historical reasoning in a computer-supported collaborative learning environment by providing representational tools. The CSCL environment we used enables pairs of students to collaborate on an historical inquiry task and in writing an essay. Sixty-five student-pairs from pre-university education participated in this research. Representational guidance was offered by a tool for the collaborative construction of an external representation, in order to select and organize information from the sources. Three different representational formats were compared with a control group: an argumentative diagram, an argument list and a matrix. As it is assumed that external representations can support both cognitive and interaction processes, we expected that using this tool would result in more historical reasoning, in the chat as well as in the text. The analyses included analyses of interaction processes in the chat, the quality of the constructed representation, the quality of the essay and the scores on the individual post test. The results indicate that each representational format has its own affordances and constraints. For example, Matrix users talked more about historical changes, whereas Diagram users were more focused on the balance in their argumentation. However, this did not result in differences in the quality of historical reasoning in the essay, nor in outcomes on the post-test.