This study investigated (a) quantitative and (b) qualitative differences between the critical thinking displayed in L1 and L2 in peer feedback discussions of Turkish EFL (English as a Foreign Language) students. High-proficiency EFL learners participated in the study on a voluntary basis after a critical thinking test. With eight high-scorer students of the test, two groups were formed with four students in each group. These students had previously taken advanced writing courses and practiced peer feedback in these courses. Still, they were given a simple feedback guide sheet and training. The participants were asked to write two argumentative essays on two different topics one week before the discussion meeting. Focusing on those essays, each group had feedback discussions in L1 (Turkish) and L2 (English), in different orders. The researcher made no interventions. The discussions were audio- and video-recorded, and transcribed for a detailed analysis. The transcriptions were coded according to a critical thinking framework that was prepared by the researcher by adapting and combining items from previously used frameworks. The findings were analyzed for the quantitative and qualitative differences between the critical thinking expressed in the two languages. The data analysis showed that critical thinking was displayed significantly more in the L1. This finding, however, is affected by the fact that participants’ total amount of talk in L1 was also more than L2 talk, therefore the quantitative difference appears to have caused by the surplus amount of total talk in the L1. The study also revealed that there were qualitative differences between the languages that critical thinking was displayed. It was speculated that the qualitative differences resulted from the ease of using the native language as well as the safety provided by the pragmatic knowledge that made communication and interaction clearer in the L1. Suggestions were made for further support to students to express their thoughts in spoken L2 more effectively, which could include more practice on giving effective peer feedback, focusing on the necessary discourse and pragmatic skills, and providing students with other discussion tasks and subject matters that invites critical thinking.