This study carries out a comparative analysis of achievement according to gender between mixed and single-sex schools in the region of Catalonia, Spain, for the subjects of Spanish, Catalan, English and Mathematics. After a brief contextualisation, a review of the main findings from international studies on differences in results for mixed schools and single-sex schools is then presented. We then outline our methodology and research-analysis plan. The study has been developed around a specific use of results obtained by students over the last year of primary school (12 years old) and over the last year of obligatory secondary education (16 years). For this comparison, the statistical technique of Propensity Score Matching was used. All segregated schools in Catalonia were chosen, representing a total of 15, of which 9 are girls-only and 6 are boys-only (with a total of 1503 students); additionally, a sample was used of 10 mixed schools, similar in terms of social make-up, that is, middle- and upper-class students (with a total of 1217 students). In general, the results corroborate international research. Results indicate that differences in achievement depending on gender in segregated or mixed schools are not related to factors of school organisation. We also come to conclusions with respect to the limitations arising from circumscribing school performance in curricular subjects, and to the need to consider further indicators within the teaching-and-learning process in terms of gender and emotional development; student attitudes and behaviour; self-concept and--most especially--teacher expectations, their teaching practices and the effects of these on self-concept and single-sex school performance.