Insight into the oxidation mechanism of microcrystalline silicon thin films has been obtained by means of Fourier transform infrared spectroscopy. The films were deposited by using the expanding thermal plasma and their oxidation upon air exposure was followed in time. Transmission spectra were recorded directly after deposition and at regular intervals up to 8 months after deposition. The interpretation of the spectra is focused on the Si–Hx stretching (2000–2100 cm-1), Si–O–Si (1000–1200 cm-1), and OxSi–Hy modes (2130–2250 cm-1). A short time scale (<3 months) oxidation of the crystalline grain boundaries is observed, while at longer time scales, the oxidation of the amorphous tissue and the formation of O–H groups on the grain boundary surfaces play a role. The implications of this study on the quality of microcrystalline silicon exhibiting no post-deposition oxidation are discussed: it is not sufficient to merely passivate the surface of the crystalline grains and fill the gap between the grains with amorphous silicon. Instead, the quality of the amorphous silicon tissue should also be taken into account, since this oxidation can affect the passivating properties of the amorphous tissue on the surface of the crystalline silicon grains.