The increasing literacy demands of senior secondary studies have been noted by government agencies and scholars both in Australia and overseas. Disciplinary differences in writing has similarly received attention, although much of the research in this area has focused on the junior school, or spanned the whole of the secondary context. Less research has been focused specifically on disciplinarity in the senior high school, or on differences within what may often be conceived as a single discipline, such as between writing in Modern and Ancient History. This paper investigates disciplinary difference in the context of senior secondary writing for Modern and Ancient History and the resulting demands on students. It focuses on the different ways that dialogism, or the negotiation of competing knowledge claims, is managed in each subject. The Systemic Functional Linguistic system of engagement is used in a discourse analysis of highly rated student writing to reveal how writers in the histories open up or close down spaces for other voices in their arguments. Analysis illustrates the ways that 'objective evaluation' is managed, illuminating one aspect of what is valued as appropriate argumentation and raising implications for the way that literacy pedagogy in the senior secondary subjects of Modern and Ancient History is understood.