The problems associated with the state of thermodynamic equilibrium between isomers of higher petroleum alkyltoluenes with a normal alkyl chain have been discussed. Approaches to the assessment by graphical means of their level of thermodynamic transformation have been proposed. The range of the thermodynamic conversion of n-alkyltoluenes has been determined. It has been shown that the conventional basic kinetic model of isomerization does not fully describe the features observed in nature. C-14, C-18 and C-22 alkyltoluenes have been synthesized and isomerized to establish their thermodynamic equilibrium in a mixture.