Abstract The effects on the photoluminescence (PL) from Si nanocrystals embedded in a SiO2 matrix (Si-NCs/SiO2) by annealing in air have been examined, and the correlation between the PL properties and the passivation of luminescence-quenching defects at the Si nanocrystal/SiO2 interface has been studied. Si-NCs/SiO2 were fabricated by annealing amorphous silicon suboxide (a-SiOx) in a N2 atmosphere at 1100 °C and the passivation of Si-NCs/SiO2 was performed by annealing Si-NCs/SiO2 in air at temperatures ranging from 400 to 800 °C for varied times. The heating of Si-NCs/SiO2 in air is very effective for passivating dangling bonds and an enhancement about 5 times in PL intensity was obtained. High-temperature passivation in air is accompanied by oxidation of Si nanocrystals. Additional N2-treatment of the passivated Si-NCs/SiO2 at elevated temperatures depassivates Si-NCs/SiO2, generating new dangling bonds which can also be passivated in the subsequent repassivation process. The effect of passivation on the PL intensity is reversible, while high-temperature passivation results in an irreversible blue shift of PL spectra. PL decay time can be indicative of passivating or generating of dangling bonds.