This mixed-methods research investigated teachers' perceptions of intrinsic factors (personal attributes of the teacher) and extrinsic factors (environmental) influencing the implementation of inquirybased science learning at township (underdeveloped urban area) high schools in South Africa. Quantitative data were collected by means of an adapted version of the Science Curriculum Implementation Questionnaire (SCIQ) ). The adapted version was renamed the Scientific Inquiry Implementation Questionnaire (SIIQ) and was administered to 186 science teachers at township schools in South Africa. The teachers at a township school were then interviewed in order to solicit in-depth information on the findings that emerged from the questionnaire analysis. The findings highlight a lack of professional science knowledge (content knowledge, pedagogical content knowledge, pedagogical knowledge, knowledge of students, educational contexts, curricular knowledge, and educational purposes) that contributes toward teachers' uncertainty in inquiry-based teaching. Also, extrinsic factors such as school ethos, professional support, resource adequacy, and time serve as significant constraints in the implementation of inquiry-based education at the school. The data collected from SIIQ provides a foundation for understanding at a high school level how factors influence the delivery of a curriculum underpinned by inquiry. # 2016 Wiley Periodicals, Inc. J Res Sci Teach 53: 598-619, 2016 Keywords: inquiry-based learning; science curriculum delivery; environmental factors One of the key imperatives in the transformation of education in South Africa is the need to provide quality education for all (Department of Education, 2001). The issues of equity and redress were foremost in transformation of the segregated education system, and the accompanying curriculum reform. The previous Apartheid education system was comprised of separate education departments for Blacks, Whites, Coloreds, and Indians, with inequitable distribution of resources for the races. Apartheid education was characterized by gross inequalities in the financing of education. Although this was reflected in all areas of school funding, the legacy of these policies is most visible in school infrastructure. A study by the Foundation for Research Development (1993) showed that the per capita expenditure for a White student was five times that for a Black students. A legacy of the apartheid policies is therefore the enormous diversity of schools in terms of the availability of physical resources. This diversity is further exacerbated by