Carbon and nitrogen are contaminant impurities in Al2O3 dielectrics grown by atomic layer deposition, leading to deleterious effects in device performance. We investigate whether these impurities can be passivated using hydrogen. The role of atomic hydrogen in the electronic properties is addressed by examining formation energies and charge-state transition levels of C-H and N-H complexes. Combined with calculated band alignment, we then assess the impact on Al2O3/semiconductor interfaces. We find that hydrogen is indeed an effective passivating agent: it removes carbon-related carrier traps and passivates negative fixed charge associated with nitrogen.