We investigate thermal stability of amorphous substoichiometric silicon‐oxide alloys with oxygen concentrations from 0 to 10 at‐%. Annealing experiments, including effective lifetime measurements, hydrogen effusion, and infrared absorption spectroscopy, yield a surface passivation on p‐type float‐zone crystalline silicon wafers with maximum effective lifetimes up to 1.5 ms. The thermal stability of the passivation increases with increasing oxygen concentration of the films. For oxygen concentrations above 5%, we observe a decrease of passivation due to the formation of voids in the material, which shows in a low temperature peak of the H effusion spectra. Annealing leads to a structural transition of the material which is accompanied by an increase in effective lifetime. The stretching mode absorptions of Si– O–Si molecular vibrations near 1000 cm–1 shift towards higher wavenumbers, both with increased oxygen concentration and upon annealing of the films. For increased oxygen concentrations, the Si–H stretching modes shift from predominantly 2000 cm–1 to 2090 cm–1(© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim) (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)