The standard (p degrees = 0.1 MPa) massic energy of combustion, in oxygen, of the crystalline 3-acetyl-2-methyl-5-phenylthiophene was measured, at T = 298.15 K, by rotating-bomb combustion calorimetry, from which the standard molar enthalpy of formation, in the condensed phase, was calculated as Delta H-f(m)degrees(cr) = -(104.3 +/- 3.1) kJ . mol(-1). The corresponding standard molar enthalpy of sublimation, at T = 298.15 K, Delta H-g(cr)m degrees(108.9 +/- 0.4) kJ . mol(-1), was derived by the Clausius-Clapeyron equation, from the temperature dependence of the vapour pressures of this compound, measured by the Knudsen effusion mass-loss technique. From the results presented above, the standard molar enthalpy of formation, in the gaseous phase, at T = 298.15 K, was derived, Delta H-f(m)degrees (g) = (4.6 +/- 3.1) kJ . mol(-1). This value, in conjunction with the literature values of the experimental enthalpies of formation of thiophene, 2-methylthiophene, and 3-acetylthiophene, was used to predict the enthalpic increment due to the introduction of a phenyl group in the position 2- of the thiophene ring. The calculated increment was compared with the corresponding ones in benzene and pyridine derivatives. (C) 2009 Elsevier Ltd. All rights reserved.