There has been considerable research into the teaching and learning of argumentation (e.g. Andrews, 2009; Sadler, 2004), focusing on strategies designed to help students to structure their written arguments. My study, however, focuses on the process of argumentation because I want to help sixth form students, aged 16-19 years old, improve the written argument in their A level History essays. The methodological approach followed was an adapted form of Design-based research, which incorporated an exploratory study, teacher trials and three case studies as part of the iterative design process. A classroom intervention was devised underpinned by design principles based in persuasive argumentation (Kuhn, 2005) and dialogic talk (Wegerif, 2012), derived from an extensive literature review, and the findings of the exploratory study. The exploratory study involved interviews with History education academics and examiners as well as classroom observations and semi-structured interviews conducted in collaboration with the teachers and students of four secondary History departments. Observations were taken of the teacher trials of the prototype intervention, whereas the data gathered from the case studies included pre and post intervention essays, audio and video recordings of the developed intervention in action, post intervention student interviews and questionnaires as well. In Case study 1 and 2, AS and A2 students’ post-intervention causation essays, when measured for argumentation, showed improvement but those whose written arguments improved the most were those students who had engaged in interactions rich in dialogic talk (Wegerif, 2012). The findings from Case study 3, which involved the integration of documentary evidence into AS History essays, were unexpected. Students found the integration of source-based evidence difficult not only during the course of the spoken argumentation but also in their written responses. Further development of the intervention is necessary to help students handle source material effectively in both the spoken and written forms of argument.