The standard (p(0) = 0.1 MPa) molar enthalpies of formation Delta(f) H-m(0) (1 or cr) at T = 298.15 K were measured by means of combustion calorimetry for 2-tert-butylphenol, 2-tert-butyl-5-methylphenol, 2-tert-butyl-4-methylphenol, 2,4-di-tert-butylphenol, 3-tert-butylphenol, 4-tert-butylphenol, 2-methyl-4-tert-butylphenol, 4-tert-amylphenol, and 4-tert-octylphenol. The standard molar enthalpies of sublimation (or vaporization) of these compounds, and also of 2-tert-amylphenol, 2,4-di-tert-amylphenol, 2-tert-octylphenol, and 2,4-di-tert-octylphenol were obtained from the temperature dependence of the vapour pressure measured in a flow system. Molar enthalpies of fusion Delta(cr)(1)H(m)(0) of the solid compounds were measured by d.s.c. Resulting values of Delta(f) H-m(0) (g) were obtained at T = 298.15 K and used to derive strain enthalpies of tert-alkylphenols. The intra-molecular interactions of the substituents were discussed in terms of deviations of Delta(f) H-m(0) (g) from the group additivity rules. The interaction energy of the tert-allcyl and OH groups in the gas phase was found to be 12.5 kL . mol(-1) for the ortho-interaction, 1.7 kJ . mol(-1) for the para-interaction, and 2.6 kJ . mol(-1) for the meta-interaction. These values provided a further improvement on the group-contribution methodology for estimation of the thermodynamic properties of organic compounds.