The chemical equilibrium of mutual interconversions of tert-amylbenzenes was studied in the temperature range 286-423 K using chloroaluminate ionic liquids as a catalyst. Enthalpies or isomerization and transalkylation of tert-amylbenzene were obtained from temperature dependences of the corresponding equilibrium constants in the liquid phase. The molar enthalpy of formation of tert-amylbenzene was measured using combustion calorimetry and was also obtained by the transpiration method. Using these experimental results, high-level ab initio methods (B3LYP and G3MP2) have been successfully tested for the prediction of reaction enthalpies for the four reactions under study. New experimental results have been used to derive a strain of tert-amylbenzene, which is useful in any additive scheme as the non-nearest-neighbor interaction which is specific for the branched alkylbenzenes.