Abstract: The mechanism of ketonization of acetic acid over SiO₂ was investigated by semiempirical quantum chemical AM1 method using the cluster approach. It has been found that the adjacent acid-base pair of the catalytic sites provokes dissociative adsorption of the acetic acid molecules resulting in the formation of surface carboxylate species. The adsorption process proceeds spontaneously. After blocking the acid-base pairs of catalyst, the new portions of acetic acid molecules interact with active species in the gas phase, converting into acyl cations. The adsorbed carboxylate species is attacked from the methyl group side by the acyl cation resulting in the formation of an acetone molecule and CO₂ by bimolecular electrophilic substitution reaction.