In the academic year of 2004-2005, the Spanish region of Madrid began to implement a bilingual educational programme (MBP hereinafter) in state schools. One of the objectives of this programme was to make the study of a foreign language (English) accessible to students from economically disadvantaged families who cannot afford private foreign language classes. Our study aims to evaluate whether students from a disadvantaged socio-economic background really do have the same probability of participating in the MBP as their more privileged peers. The analysis use the PISA 2015 database which corresponds to the representative sample of the Community of Madrid in Spain, with added administrative information supplied by the Madrid Regional Ministry of Education concerning the identification of bilingual and non-bilingual schools. Using these data, we estimate a logit model directed at identifying which factors explain the choice by students of whether to attend a bilingual state school. The results obtained reveal that the probability of attending a bilingual school is higher for students belonging to socio-economically and culturally better-off households. This suggests that the MBP could be fostering segregation within the state education sector in Madrid.