To make informed decisions has been acknowledged as an essential ability to negotiate socioscientific issues. However, many young people show an inadequate understanding of how to make well-informed decisions, particularly in contexts that are connected to environmental problems. This paper aims to explore the effectiveness of an environmental science competition (BundesUmweltWettbewerb, BUW) to foster students’ socioscientific decision-making. Two different instruments, a paper-pencil test (N = 196 students) and retrospective interviews (N = 10 students), have been used in two successive studies. In addition, both of the applied instruments are investigated theoretically using the “assessment triangle” of the National Research Council (National Research Council, Knowing What Students Know, 2001) as a framework. The results of our studies indicate that participating in the environmental science competition predominantly fosters students’ socioscientific decision-making in its pre-selectional phase. We further argue that promoting the selectional phase of decision-making requires explicit and instructional guidance. With respect to the assessment of socioscientific decision-making, a focus on either structural (decision-making strategies) or contextual (decision content) conditions is argued. Outcomes are discussed in terms of theoretical and practical implications.