In situ FTIR spectroscopy has been used to investigate the adsorption of acetonitrile at room temperature onto zirconia activated at various temperatures (423, 723 and 873 K), attention being focussed on the sample pretreated at 423 K. Two acetamide monoanions (CH₃CONH^–), called α and β species, were spectroscopically evidenced with main bands, respectively, at 1169, 1432, 1590, 3320 cm^–1 and at 1196, 1471, 1557, 3340 cm^–1. The use of ¹⁸O- or D-labelled compounds gave poor results as far as the vibrational mode description was concerned, except for the δ(NH) mode. However, the H → D substitution was very useful for determining mechanisms. It was proposed that the β-type acetamide species results from H₂O displacement from the surface and CH₃CN adsorption in acidic Lewis sites, followed by a hydroxylation of this latter species by H₂O readsorption. The formation of the α-type acetamide species was thought to be due to a direct interaction of acetonitrile with basic OH groups. Upon heating at a temperature higher than 373 K and in the presence of water vapour, acetamide species were hydrolysed into acetate species.