We propose two contrasting types of student decision-making based on social and cognitive psychology models of separate mental processes for problem solving. Informal decision-making uses intuitive reasoning and is subject to cognitive biases, whereas formal decision-making uses effortful, logical reasoning. We explored indicators of students' formal decision-making in a socioscientific issue-focused general science post-secondary course that used a seven-step decision-making framework based in decision-science models. We found that students' value orientations predicted students' stance towards biofuels at the beginning but not at the end of the course, indicating some students' post-survey opinions were less rooted in affective value-judgments. Themes in student personal reasoning about biofuels were primarily focused on environmental and economic dimensions of the issue and revealed a greater awareness of alternative options and potential consequences of biofuel use at the end of the course. We observed a decrease in students' emotive arguments and an increase in students' justificatory arguments about biofuel use. A formal decision-making framework may be a useful tool in supporting students' formal, reasoned approach to deciding what we should do about complex socioscientific issues.