The purpose of this study is to explore the factors that affect students' decision-making in an SSI (socio-scientific issue) about nuclear power and energy supply. The study explores students' skills in SSI argumentation and aims to clarify the relationship between values, knowledge and experiences in their SSI decision-making. Nine students grade nine participated. Their argumentation and decision-making processes were followed, the participants wrote reports on their decision-making, and data were collected during multiple stages of an instructional module, the Six-Step SSI Instructional Module. The different arguments used by the students were analysed using the framework of the SEE-SEP-model. The analysis focused on the evaluation skills demonstrated by the students during the exercise and the relationships between the knowledge, values, and experiences that they used in their argumentation. Although all the students had access to the same information and agreed on the factual aspects of the issue, they came to different decisions, the difference depending on their background knowledge, values, and experiences. The assessment framework considers the quality indicators presented in the research and focuses on both the content and the structure that can be observed in students’ SSI argumentation and is meant to function as a tool for identifying quality indicators. The implication for SSI teaching and learning is discussed.