The standard (p(o) = 0.1 MPa) molar enthalpies of formation Delta(f)H(m)(o)(1, or cr) at the temperature 298.15 K were measured by means of combustion calorimetry for 2,3,6-tri-methylphenol, 2,4,6-tri-methylphenol, 2,6-di-iso-propylphenol, 2-tert-butyl-6-methylphenol, 2-tert-butyl-4,6-di-methylphenol, 2,6-di-tert-butylphenol, 2,6-di-tert-buty-4-methylphenol, and 2,4,6-tri-tert-butylphenol. The standard molar enthalpies of sublimation (or vaporization) of these compounds were obtained from the temperature dependence of the vapor pressure measured in a flow system. Molar enthalpies of fusion Delta(cr)(1), H-m(o) of the solid compounds were measured by d.s.c. Resulting values of Delta(f)H(m)(o)(g) were obtained at the temperature 298.15 K and used to derive strain enthalpies of alkylphenols. The intramolecular interactions were discussed in terms of the enthalpic contribution of the substituents. The buttress interaction of tert-alkyl groups with hydroxyl t-Bu-OH-t-Bu was found to be 5.0 kJ.mol(-1). Other buttress effects of methyl, iso-propyl-, and tert-alkyl substituents with the OH-group were found to be negligible within +/-(1 to 2) kJ.mol(-1) These values provided a further improvement on the group-contribution methodology for estimation of the thermodynamic properties of organic compounds.