This study aims to explore students' argumentation and decision-making relating to an authentic socioscientific issue (SSI)--the problem of environmental toxins in fish from the Baltic Sea. A multi-disciplinary instructional module, designed in order to develop students' skills to argue about complex SSI, was successfully tested. Seven science majors in the final year of their upper secondary studies participated in this study. Their argumentation and decision-making processes were followed closely, and data were collected during multiple stages of the instructional module: group discussions were audio recorded, the participants wrote reports on their decision making, and postexercise interviews were conducted with individual students. The analysis focused on the skill of evaluation demonstrated by the students during the exercise and the relationships between the knowledge, values, and experiences that they used in their argumentation. Even though all of the students had access to the same information and agreed on the factual aspects of the issue, they came to different decisions. All of the students took counter-arguments and the limitations of their claims into account and were able to extend their claims where appropriate. However, their decisions differed depending on their background knowledge, values, and experiences (i.e., their intellectual baggage). The implication to SSI teaching and learning is discussed.