In order to cultivate students to be able to participate in public affairs and make decisions about socioscientific issues (SSI), a web-based module was designed for students to collaboratively engage in the decision-making (DM) process. This study attempted to identify students' discourse characteristics that might lead to formulating an evidence-based decision on SSI. Twenty-nine Grade 10 students were randomly divided into eight groups of three or four. The transcribed data of one case from each performance level were compared to investigate the interplay between groups' DM performances and discourse characteristics. The results showed that the group that gained a high score on the DM group worksheet engaged in the metacognitive discussion for planning procedures of the module tools and in the conceptual exchanges to accomplish the tasks. The members of this group could initiate and extend ideas, provide prompts, and confirm or reject each other's ideas, resulting in sustained interactive dialogs that allowed them to learn from one another. This indicated that students need to be encouraged to clarify the task goals, plan procedures, monitor their performance, and exchange their ideas actively. The implications of how collaborative discourse promote students' SSI DM performance, and the better design and enactment of SSI modules are discussed.