This study aims to explore students’ argumentation and decision-making relating toan authentic socioscientific issue (SSI)—the problem of environmental toxins in fish from theBaltic 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 finalyear of their upper secondary studies participated in this study. Their argumentation anddecision-making processes were followed closely, and data were collected during multiplestages of the instructional module: group discussions were audio recorded, the participantswrote reports on their decision making, and postexercise interviews were conducted withindividual students. The analysis focused on the skill of evaluation demonstrated by thestudents during the exercise and the relationships between the knowledge, values, andexperiences that they used in their argumentation. Even though all of the students had accessto the same information and agreed on the factual aspects of the issue, they came to differentdecisions. All of the students took counter-arguments and the limitations of their claims intoaccount and were able to extend their claims where appropriate. However, their decisionsdiffered depending on their background knowledge, values, and experiences (i.e., theirintellectual baggage). The implication to SSI teaching and learning is discussed.