The aim of this paper is to test whether the distribution of students by social, cultural and racial characteristics is homogeneous between Spanish public schools (PS) and publicly subsidised private schools (PSPS) or whether segregation exists between the profile of pupils attending each type of school. The theoretical framework is based on the contributions of researchers into school choice policies, while the empirical application uses a 2005 questionnaire answered by the final year secondary school students of the Spanish region of Aragón. We quantify the degree of internal segregation within each sector (PS and PSPS) and estimate a probit model in order to discover which factors determine the choice of a PSPS. We conclude that the distribution of pupils between PS and PSPS follows a clear socioeconomic pattern which favours privately owned schools. Our study offers an additional result, namely, that cream-skimming processes are more recurrent within the publicly subsidised sector, which is shown to be far more selective than the public sector in its distribution of pupils. Finally, it is found that the higher the socioeconomic status, the higher the probability of choosing PSPS, suggesting that the segregation found in this paper may be caused partly by the choice patterns of Spanish families.