Developing students' source-based argument writing skills is a vital educational goal for the 21st-century information society. Consequently, researchers and educators continually seek ways to understand and improve students' capacities for advancing arguments and synthesizing multiple documents, texts, or sources in a range of subject areas in secondary schools. This study examined differences between middle and high school students' argument essays (N = 207) in multiple dimensions of source-based argument writing in history, the dimensions writing in history, and the relations of identified dimensions to overall writing quality. Using multivariate analysis of covariance, middle and high school students' writing significantly varied in areas of writing related to language use, the presentation of ideas, and evidence use. Their writing varied less so for skills related to historical thinking, indicating a lack of development in these skills across secondary school. Findings from confirmatory factor analysis and structural equation modeling showed a bifactor model with a general factor and 4 specific factors--Presentation of Ideas, Evidence Use, Language Use, and Historical Thinking--best represented writing in this genre, with the general factor strongly predicting holistic writing scores. Implications for both research and educational practice are discussed, including the importance of attending to developmental variation in discrete writing skills.