The surface passivation properties of thermal-SiO2/plasma-enhanced chemical vapor deposition SiN stacks on 2.5 Ω cm n-type Czochralski silicon substrates have been investigated. By annealing these stacks in air we achieve surface recombination velocities (SRV) lower than 2.4 cm/s for thin SiO2 layers. We find a clear correlation between the thickness of the oxide layers and the annealing duration. We also show that the absolute passivation quality of the SiO2/SiN stacks correlates to the SiO2 thickness. We find that the SRV increases with increasing oxide thickness. We also present data of the surface passivation of these SiO2/SiN stacks after storage in the dark for several weeks.