A study of the adsorption sites of thoria has been made using scanning transmission electronic microscopy (STEM) and Fourier-transform infrared spectroscopy (FTIR). The STEM study showed that the thoria surface was composed mainly of (110)(211) and (111) faces in about equal proportions. The infrared study of activated ThO₂ at 873 K indicated the existence of two remaining hydroxyl groups at 3660 and 3510 cm^–1. With water adsorption, a third one appeared at 3745 cm^–1 which was thermally less stable. Methanol adsorption gave rise to three kinds of species: (i) one reversibly adsorbed at room temperature and linked by hydrogen bonds in which thoria acts as a proton receiver; (ii) two methoxy species characterized by ν(CO) bands at 1127 cm^–1(species I) and 1060 cm^–1(species II). Species I desorption bands led to methanol whilst species II, much more thermally stable, gave methanol, dimethyl ether and carbon monoxide. Species I, which corresponds to the hydroxyl species at 3745 cm^–1, are of the Th—OMe (Th—OH) type, and are formed on the (110) faces. Species II, which correspond to hydroxyl species at 3660 cm^–1, are of the [graphic omitted] type, and are formed on the (211) faces. The third hydroxyl species, characterized by the ν(OH) band at 3510 cm^–1, is probably due to species of the [graphic omitted] type appearing on (111) faces. The stability and reactivity of the methoxy species are shown to depend on their structure, and therefore on the local arrangement of the surface.