Sustainability has become a prominent theme in society and can be considered as an integral part of scientific citizenship. This study investigates to what extent the production, use and re-use of (bio)plastics initiates students' reasoning and to identify the kind of content knowledge students put forward. The structure of students' arguments was mapped according to Toulmin's model of argumentation, i.e., claim, data, warrant & backing and qualifier & rebuttals. Students (N = 27, grade 10 & 11) participated in groups of three. The students were introduced to the topic of the production, use and re-use of plastics by watching a video, answering questions, reading articles and having interviews and group discussions. Students were prompted to argue on the sustainability of bioplastics and fossil-based plastics. The results show that students frequently used arguments related to preventing pollution, designing to recycle and designing to degrade. However, themes such as avoiding waste, origin of energy and materials, energy efficiency and costs were rarely used or even absent in students' reasoning. Overall, the students' reasoning contained all of Toulmin's categories, and especially the increase in the number of qualifier & rebuttals is interpreted as an indication of awareness of the complexity of the issue at hand. This study underlines that students are able to bring in relevant scientific knowledge when confronted with a suitable sustainability issue, but also more societally oriented arguments enriched their perspective. Implications for the design of interventions aiming to engage students in life cycle analysis (on plastics) are discussed.