The question of the mechanism of formation of carbonium ions on Lewis or Bronsted acid centres remains unclear. Studies of the chemical conversion of a model cumene hydrocarbon and also its chemisorption on a cracking catalyst by means of gas-liquid chromatography have shown that carbonium ions are formed from isothermally desorbable cumene chemisorbed on Lewis acid centres. The proton sources are the OH groups chemisorbed on the catalyst.