Teachers cannot escape being learners if they are to be proficient practitioners. This study interrogates the effects of a professional development programme on mathematics teachers' pedagogical practices in South African primary schools. Using critical theory as the illuminating lens, this qualitative study, which is cast within the participatory action research paradigm, examined mathematics teachers' classroom practices before and after a professional development programme. Data were collected from 41 mathematics teachers during the pre-intervention cycle, the intervention programme, and the post-intervention cycle. Document analysis, focus group discussion, free attitude interviews and lesson observations were the data collection tools. In the pre-intervention cycle, mathematics teachers blamed learners for errors in solving complex mathematical problems, without reflecting on their own pedagogical shortcomings. The instructional intervention programme enabled teachers to reflect more critically on their teaching methods. It also enriched their mathematical content knowledge and pedagogical skills in using manipulatives. Post-intervention lesson observations showed some improvements in the teachers' classroom practice as they reaped the benefits of the professional development programme. This study begins to demonstrate that, if properly organised, professional development activities can assist in filling gaps in teachers' content mastery and pedagogical skills; ultimately improving learner performance. The implications of this study are that teacher professional development, if aligned to teachers' pedagogical needs, can assist in improving classroom practice and is not a waste of resources as some previous studies have claimed.