This study examined 10th-grade students' use of theory and evidence in evaluating a socio-scientific issue: the use of underground water, after students had received a Science, Technology and Society-oriented instruction. Forty-five male and 45 female students from two intact, single-sex, classes participated in this study. A flow-map method was used to assess the participants' conceptual knowledge. The reasoning mode was assessed using a questionnaire with open-ended questions. Results showed that, although some weak to moderate associations were found between conceptual organization in memory and reasoning modes, the students' ability to incorporate theory and evidence was in general inadequate. It was also found that students' reasoning modes were consistent with their epistemological perspectives. Moreover, male and female students appear to have different reasoning approaches.