THIS paper reports research conducted as part of the Single-Sex Education Pilot Project (SSEPP) which was undertaken in 1993–1994 in Western Australia. The project involved the implementation of single-sex science and/or mathematics in ten coeducational secondary schools. Unlike previous similar initiatives, the SSEPP included professional development for the teachers involved and systematic monitoring. The paper presents an analysis of teachers' reactions to and perceptions of the SSEPP. It gives voice to teachers' views of the major issues which arose during the project, concerning outcomes for students, school organisation, teacher professional development, community response and sustainability of the change.