Decision making about socioscientific issues is an important aspect of modern science education worldwide. Among the many topics that represent socioscientific issues, issues relating to the sustainable development of our environment are one crucial topic. However, difficulties exist with respect to the assessment of teaching outcomes related to these issues. This paper presents results from two quantitative studies that were conducted to develop a test instrument that measures students' use of decision-making strategies in situations relating to sustainable development. Data were analyzed using the Rasch partial credit model. Analyses concerning reliability and validity showed that the developed instrument provides an adequate measure of students' use of decision-making strategies. Analyses indicated that the test instrument can be used among different age groups and that decision-making competence increases with respect to years of education. Most elaborate strategies were characterized by the use of trade-offs, the ability to weigh decision criteria, and the ability to reflect on the structure of decision-making processes. In contrast, baselevel strategies were characterized by the use of cutoffs and an absence of elaborate metareflection. The results provide a valuable starting point for analyzing and fostering students' decision-making competence in the science classroom. (Contains 3 figures, 2 tables, and 3 footnotes.)