Teaching mathematics through themes has been praised for relating mathematics to real life situations. However, research shows that the implementation of teaching mathematics thematically has not been widely adopted. In the present study, instructional, curricula and organisational factors that teachers perceive as obstacles to the implementation of a thematic approach to teaching mathematics were explored. Findings from interviews with 10 secondary mathematics teachers who were implementing a curriculum requiring a thematic instructional approach are reported. The findings of the study extend the literature concerning teachers' beliefs and practices in the teaching of mathematics, and broaden understandings of the issues surrounding the implementation of a thematically taught mathematics curriculum.