This study investigated argumentation patterns resulting in teacher-guided and peer-guided group discussions on four socioscientific-issues (SSI). Two groups, each including five students from grade 7, studied on a different SSI during four weeks. Discussions within both groups were observed, videotaped, and analyzed qualitatively. After four weeks, group interviews were conducted. The results showed that teacher-guided group presented more complex argumentation patterns than peer-guided group. Both groups supported their claims with scientific and non-scientific evidence. But teacher-guided group presented the evidence deeply. The results suggested that teachers should have the related pedagogical skills to put argumentation into practice and to explore the students' skills in constructing arguments in the context of SSI. The implications for science educators and researchers were discussed. [Paper presented at the International Conference on Education in Mathematics, Science & Technology (ICEMST) (Konya, Turkey, May 16-18, 2014).]