International student mobility (ISM) prepares young people for the challenges of global and multicultural environments. However, disadvantaged students have lower participation rates in mobility schemes and, hence, benefit less from their positive impacts on career progression. Therefore, policymakers aim to make mobility programs more inclusive. Nevertheless, it is far from clear how policy design can achieve this aim. This study investigates factors driving inequality in international student mobility uptake. The study's novelty is twofold: first, in contrast to most existing studies it does not only investigate individual but also university characteristics as possible drivers of unequal uptake. This is possible due to the use of rich graduate survey and administrative data merged with university-level European Tertiary Education Register (ETER) data. Second, the study compares results across four European countries. Results show that the socio-economic mobility gap remains still sizable even when taking university characteristics into account. However, universities matter considerably and especially student compositions in terms of socio-economic background and ability contribute to unequal ISM uptake. As a consequence, intergovernmental policies should aim to distribute grants and mobility opportunities more equally across all universities, independent of their student composition.