The effect of composition may be strong in systems where students are systematically sorted based on their socio-economic background. This paper aims to model the differential effect of class composition on pupils' achievement in Belgium (French-speaking Community), France, Spain and Portugal. Multilevel models are consequently tested on the Progress in International Reading Literacy Study 2011 data (20,830 pupils in 1139 classes). Our results suggest that socio-economic composition does not have an equivalent effect on pupil achievement in the four countries included in our analysis: its effect is strong in the French-speaking Community of Belgium and France but smaller in Spain and Portugal.