This study examined the influence of the mathematics profiles of secondary school pre-service mathematics teachers on their instructional behaviour. The mathematics profile construct was determined with respect to four components, namely, subject matter knowledge, pedagogical content knowledge, conceptions of mathematics and beliefs about the teaching and learning of mathematics. The instructional behaviour construct was studied with regard to participants’ use of a traditional versus reform approach to teaching, and whether learners were afforded an authoritarian versus democratic style of learning. Social constructivism formed the epistemological underpinning. The context for the study was a Post Graduate Certificate in Education (PGCE) course at a university in South Africa. The study adopted a qualitative post-hoc research approach of seven case studies. The final portfolios submitted by participants as part of their PGCE course were used as the main source of data. Through participant and researcher reflections, a visual representation of each participant’s mathematics profile and instructional behaviour was constructed. These were then compared in within-case and cross-case comparisons. Findings indicated that the mathematics profiles of pre-service mathematics teachers have an influence on either enabling or constraining the development of their instructional behaviour. An improvement in the pedagogical content knowledge of mathematics teachers without positive changes in their conceptions and beliefs and the quality of their reflections and subject matter knowledge does not result in reformed instructional behaviour. The mathematics profile as a package needs to be developed in order for pre-service mathematics teachers to make the required changes in their instructional behaviour towards a more reform-orientated approach to teaching and learning of mathematics.