The standard (p(o) = 0.1 MPa) molar enthalpies of formation Delta fH(m)(o) (l or cr) at the temperature T = 298.15 K were measured using combustion calorimetry for tert-butylbenzene, 4-methyl-1-tert-butylbenzene, 3,5-dimethyl-1-tert-butylbenzene, 5-methyl-1,3-di-tert-butylbenzene, and 1,4-di-tert-butylbenzene. The standard molar enthalpies of vaporization Delta(l)(g)H(m)(o), or sublimation Delta(cr)(g)H(m)(o), of these compounds, and also of 3-methyl-1-tert-butylbenzene, 1,3-di-tertbutylbenzene, and 1,3,5-tri-tert-butylbenzene were obtained from the temperature function of the vapour pressure measured in a flow system. Enthalpies of fusion Delta(cr)(l)H(m)(o) of solid compounds were measured by d.s.c. Strain enthalpies of branched alkylbenzenes were derived from the measured enthalpies of formation of the gaseous compounds. These experimental results, together with the data available from the literature, provided a quantitative understanding of the interrelations of structure and energetics of alkylbenzenes, and a further improvement on the group-contribution methodology for estimation of thermodynamic properties of organic compounds.