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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 (1 10) (21 1) and (1 11) faces in about equal proportions. The infrared study of activated Tho, at 873 K indicated the existence of two remaining hydroxyl groups at 3660 and 3510 m -l . With water adsorption, a third one appeared at 3745cm-' 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 v(C0) bands at 1127 cm-l (species I) and 1060 cm-l (species 11). Species I desorption bands led to methanol whilst species 11, much more thermally stable, gave methanol, dimethyl ether and carbon monoxide. Species I, which corresponds to the hydroxyl species at 3745 cm-l, are of the Th-OMe (Th-OH) type, and are formed on the (1 10) faces. Species 11, which correspond to hydroxyl species at 3660cm-l, are of the ' 0-Me / Th Th Th 0-H) type, and are formed on the (21 1) faces. The third hydroxyl species, characterized by the v(0H) band at 3510 cm-l, is probably due to type appearing on (1 1 1) faces. The stability species of the