The equilibrium of six positional isomerization reactions in the systems of sterically unhindered tert-butylbenzenes (TBB) and tert-butyltoluenes (TBT) was experimentally studied in the liquid phase over a wide temperature range. The thermodynamic characteristics of the reactions were obtained, their interpretation was given, and the enthalpy of interaction of the (1-TB)-(3-TB) type between the substituents in alkylbenzene molecules was determined on the basis of the experimental and literature data.