Teachers may face considerable challenges when implementing socio-scientific issues (SSI) in their classroom practices, such as incorporating student-centred teaching practices and exploring knowledge and values in the context of socioscientific issues. This year-long study explores teachers' reflections on the process of developing their classroom practices when implementing SSI. Video-recorded discussions between two upper secondary school science teachers and an educational researcher, grounded in the teachers' reflections on their classroom practices, provided data for the analysis. The results show that during the course of the implementation the teachers enhanced their awareness of the importance of promoting students' participation and supporting their independence as learners. However, the results also suggest a conflict between the enactment of a student-centred classroom practice and the achievement of intended learning goals. In order to accept the challenge of implementing SSI in the classroom, it is suggested that it is essential for teachers to build strategies, which integrate dialogue about learning goals.