Research focusing on students’ argumentation in socioscientific issues (SSI) shows that students tend to base their arguments on values rather than knowledge. This study explores Swedish upper secondary chemistry students’ written argumentation. The data consists of student texts written at the end of an intervention designed to develop skills related to high quality argumentation. The results show that after being taught about argumentation and the context of SSI, students mainly base their arguments on content knowledge. Value justifications are present in students’ texts, but constitute a smaller proportion. Beside content knowledge- and value justifications, we found a third category – “reasoning” – in which students draw conclusions, or make predictions of future events, to support or refute a claim. The justifications in the argumentative texts include a breadth of subject areas in which chemistry knowledge plays an important role. This study suggests that content knowledge constitutes an important part in student argumentation.