dThe concentrations of diisopropylnaphtalene (DIPN) isomers in experimental equilibrium mixtures obtained on amorphous aluminosilicate and HY zeolite have been measured by GC method. The total energy and Gibbs free energy of the isomers were calculated by DFT method at the B3PW91/3-21G level. Comparison of the populations based on the DFT energies with the experimental concentrations of DIPN isomers are in good agreement. This fact can suggest that isomerization is mainly determined by thermodynamic equilibrium.