In Africa, 5.8 per cent of enrolled tertiary students go outside their homelands for tertiary study. No other world region has this high a share of outbound student mobility. In this study, I examined why African countries have larger student outflows than other regions and, in particular, I considered the importance of tertiary education capacity in the region for student mobility. I evaluated the determinants of student outflows from African countries for three different measures: the total number of tertiary students abroad, the percentage of the tertiary age cohort studying abroad and the percentage of total enrolled students abroad. In addition to showing that country rankings differ on these mobility measures, the findings indicate that their determinants also differ. The study premise was that student outflows should be lower from countries that have a greater supply of tertiary training capacity and that thesis received strong support in models that estimated the percentage of total enrolled students abroad. In models for that outcome, student outflows were also larger if countries had high tertiary demand and populations under 2 million. The findings for models that estimated total numbers abroad and share of the tertiary cohort abroad were similar after controlling for interactions between tertiary education supply and GDP per capita. In addition, population size and per capita GDP were stronger correlates of student mobility in those models, which suggests that it is more difficult for education supply and demand measures to account for student outflows when crude outflow measures are used. I concluded that strengthening tertiary education supply at home would be a cost-effective way for African governments to increase their human capital and reduce brain drain losses.