Three tetramethylbenzene isomers, together with penta- and hexamethylbenzene have been studied by sublimation or vaporisation calorimetry. This work enabled us to determine for each substance its enthalpy of sublimation or vaporisation. From all the results of the enthalpy of vaporisation for methyl derivatives of benzene, it was possible to elaborate a linear relationship between this thermodynamic function and the number of methyl groups per molecule of this series.