Language is essential for making meaning in written communication, and argument writing is a key genre of schooling to which language contributes rich resources for constructing different types of arguments. Despite being a challenging language practice, argument writing research lacks investigation into the language demands of this writing genre. Using an explanatory sequential mixed methods approach, the present study explored the role of language in adolescent students' argument writing through the lens of the relationship between argumentation features (reasons, counterarguments, rebuttals) and students' use of epistemic modality devices (EMDs) -- linguistic tools that express the author's knowledge of and belief about the possibility of whether a state of affairs is true (e.g., 'must', 'maybe'). Argument writing samples from 115 upper elementary students were analyzed to explore the relationship between EMDs and written argumentation. Descriptive analysis showed that reasons and modal auxiliaries that indicated certainty ('will', 'would') were most frequently employed by the students. Multiple regression analyses revealed a positive, significant relationship between number of reasons in students' writing and their EMD usage. Subsequent qualitative content analysis identified two salient patterns of how EMDs assisted with reason construction, (1) predicting consequences, and (2) speculating causes of behaviors. The findings emphasize the close relation between language and argumentation and hence highlight the importance of the explicit instruction of language features pertinent to argumentative discourse.