This pilot study focuses on the potential of Evidence-based Dialogue Mapping as a participatory action research tool to investigate young teenagers' scientific argumentation. Evidence-based Dialogue Mapping is a technique for representing graphically an argumentative dialogue through Questions, Ideas, Pros, Cons and Data. Our research objective is to better understand the usage of Compendium, a Dialogue Mapping software tool, as both (1) a learning strategy to scaffold school pupils' argumentation, and (2) as a method to investigate the quality of their argumentative essays. The participants were a science teacher-researcher, a knowledge mapping researcher and 20 pupils, 12-13 years old, in a summer science course for "gifted and talented" children in the UK. This study draws on multiple data sources: discussion forum, science teacher-researchers' and pupils' Dialogue Maps, pupil essays and reflective comments about the uses of mapping for writing. Through qualitative analysis of two case studies, we examine the role of Evidence-based Dialogue Maps as a mediating tool in scientific reasoning: as "conceptual bridges" for linking and making knowledge intelligible; as support for the "linearization task" of generating a coherent document outline; as a "reflective aid" to rethinking reasoning in response to teacher feedback; and as a "visual language" for making arguments tangible via cartographic conventions. (Contains 2 tables, 17 figures and 2 notes.)