The cyclodimerization of alpha-methylstyrene in the presence of Bronsted and Lewis acids was realized. The dependence of the cyclic alpha-methylstyrene dimer yield on the catalyst nature and structure was established. It was shown that the activity of the catalysts is determined by the strength of their acid sites, and the selectivity for the cyclic alpha-methylstyrene dimer depends on the nature of the anion (homogeneous acids) or the porous structure (heterogeneous catalysts). Sulfuric acid and silicophosphate exhibit the highest selectivity in the cyclodimerization of alpha-methylstyrene into 1,1,3-trimethyl-3-phenylindane.