The gas-phase enthalpies of formation for a set of ortho-substituted alkylbenzenes were obtained from CCSD(T*)-F12 and WI-F12 calculations. Most values are in keeping with available experimental data. The gas-phase enthalpies of formation of 1-ethyl-2-propylbenzene, 1-ethyl-2-isopropylbenzene, 1,2-diisopropyl-benzene, 1,2,4-triethylbenzene, and 1,2,4,5-tetraethylbenzene, for which no experimental data are available, were determined as -46.0, -46.8, -68.7, -76.9, and -116.8 kJ mol(-1), respectively (estimated error bar +/- 4 kJ mol(-1)). The whole set of experimental and theoretical values are in good agreement with the estimates obtained using the extended Laidler bond additivity (ELBA) method. This agreement supports the approximation used in ELBA that interactions between ortho alkyl groups (other than tertiary alkyl groups) have roughly the same magnitude as a methyl-methyl interaction.