The catalytic activity of Beta zeolites, modified with magnesium, boron, and phosphorus compounds, was studied using benzene and toluene alkylation by ethylene. The concentrations of different types of acid centers in these zeolites were measured using IR spectroscopy of OH groups and adsorbed CO. Ortho-, meta-, and para-isomer distributions of the primary alkylation products of toluene are determined by aromatic substitution rates. But, if the conditions for further isomerization of the product are created by increasing either the reaction temperature or the concentration of strong acid centers in the catalyst, the selectivity ratio among the isomers will be changed and approach the thermodynamic equilibrium distribution.