This thesis focuses on upper secondary students’ use of resources in their supporting reasons when arguing socioscientific issues (SSIs). The skills of argumentation have been emphasized in science education during the past decades and SSIs are proven a good context for learners to enhance skills of argumentation and achieve the goal of scientific literacy. Research has shown that supporting reasons from various resources are embedded in students’ argumentation on SSIs, and also that multi-perspective involvement in reasoning is important for the quality of argumentation. To explore the reasons used by students in arguing about SSIs in this thesis, the SEE-SEP model was adopted as an analytical framework. The SEE-SEP model covers the six subject areas of sociology/culture, economy, environment/ecology, science, ethics/morality and policy, which are connected to the three aspects of knowledge, value and personal experience. Two studies covering four SSIs (global warming, GMO, nuclear power and consumption) explore how students construct arguments on one SSI topic chosen by them. In paper I, I investigated students’ use of resources in their informal argumentation and to what extent students made use of knowledge. The results showed that students used value to a larger extent (67%) than knowledge (27%). I also found that the distribution of supporting reasons generated by students varied from the different SSIs. In paper II, I explored students’ use of resources in relation to students’ study background (science majors and social-science majors) and gender. The results showed that social-science majors and females generated more numbers of reasons and also showed a larger amount of multi-disciplinary resources in their supporting reasons. From the findings of this thesis, the SEE-SEP model was established as a suitable model used to analyze students’ resources of supporting reasons while arguing about SSIs. Furthermore, the potential for applying the SEE-SEP model in teachers’ SSI-teaching and students’ SSI-learning is suggested. The implications to research and teaching are also discussed.