The purpose of this study was to identify the testable beliefs of three secondary mathematics teachers, though for reasons of space only two are reported here. The teachers were involved in a collaborative teacher-led professional development program. The aims of this program were to enable the teachers to reflect on their practice, investigate issues of concern and by collaboration alter their instructional practice. The teachers’ beliefs were identified and several inconsistencies highlighted. This study was able to identify testable beliefs for each teacher and a further study intends to investigate whether these inconsistencies once recognised by the teachers can be tested through teacher-led professional development.