We analyse whether formal education and on-the-job skill acquisition are substitutes or complements for a sample of Flemish school-leavers. Skill acquisition is measured directly through subjective assessments. We find that higher educated workers are more likely to acquire additional skills. While this is primarily explained by between-occupation effects, also within-occupation effects are revealed. Undereducated workers have lower overall skill acquisition probabilities than adequately educated workers in similar occupations; overeducated workers with a vocational degree acquire fewer transferable or general skills than their adequately educated colleagues. Overeducated workers also acquire fewer additional skills than adequately educated workers with similar educational backgrounds.