The role of hydrogen in Si surface passivation is experimentally identified for Al2O3 (capping) films synthesized by atomic layer deposition. By using stacks of SiO2 and deuterated Al2O3, we demonstrate that hydrogen is transported from Al2O3 to the underlying SiO2 already at relatively low annealing temperatures of 400¿°C. This leads to a high level of chemical passivation of the interface. Moreover, the thermal stability of the passivation up to 800¿°C was significantly improved by applying a thin Al2O3 capping film on the SiO2. The hydrogen released from the Al2O3 film favorably influences the passivation of Si interface defects.