This study examined junior high school students’ knowledge about the process of argumentative writing and whether discourse knowledge predicted students’ writing performance. Participants were 26 ninth grade students (aged 13–15). Students wrote an argumentative text and responded to questions assessing their declarative knowledge about writing in general, and their declarative, procedural and conditional knowledge about argumentative writing in particular. Students’ responses describing the argumentative writing process were mainly centred on substantive and production procedures, on abilities associated with writing and on seeking assistance for text composing. Descriptions were mainly focused on opinion writing, omitting the importance of taking an argumentative position and of using evidence to support arguments and counterarguments. A reduced number of students considered the need to accommodate their writing to a potential reader. In addition, knowledge of substantive procedures and knowledge of reader’s fit were significantly correlated with writing quality and planning complexity.