This study examines how the use of a science inquiry activity in an environmental socio-scientific issue (SSI) impacts pre-service teachers' argumentative practice in two ways: social negotiation and epistemic understanding of arguments. Twenty pre-service science teachers participated in this study as a part of their science methods class. Small group discussions, while participating in an SSI debate, before and after engaging in a science inquiry activity, were collected as a main data source. The data were analyzed by an analytic framework adapted from both Toulmin's (1958) model of argument structure and Walton's (1996) reasoning scheme. The results indicate that the use of a science inquiry activity during SSI debate not only affects the teachers' social negotiation patterns, but also enhances their epistemic understanding. This study suggests that incorporating a science inquiry practice into an SSI debate has the potential to improve students' disciplinary engagement and the quality of their argumentative practice.