The i.r. spectra of acetone adsorbed on two samples of magnesia of different acidity were correlated with the products found in thermal desorption (300–1000 K). Desorption of methane was observed with the sample of higher basicity, MgO-II, in the high-temperature range and desorption of acetone at lower temperatures. With the other sample, MgO-I, acetone was the only species desorbed. The higher basicity of MgO-II is reflected in the i.r. spectra by a lower interaction strength with the surface hydroxyl groups, a higher downward shift of the CH wavenumber and the appearance of a CO band at 1660 cm–1. At the temperature of methane formation, CH bands assigned to acetate structures appear in the infrared spectrum. A mechanism of methane formation accounting for the importance of basic sites is proposed.