Values of the condensed phase standard (p = 0.1 MPa) molar enthalpy of formation for 2'- and 4'-methylacetophenones were derived from the standard molar energies of combustion, in oxygen, at T = 298.15 K, measured by static bomb combustion calorimetry. The values of the standard molar enthalpy of vaporization, at T = 298.15 K, were measured by Calvet microcalorimetry. Combining these two values, the following enthalpies of formation in the gas phase, at T = 298.15 K, were then derived: 2'-methylacetophenone, -(115.7 +/- 2.4) kJ . mol(-1), and 4'-methylacetophenone, -(122.6 +/- 2.4) kJ . mol(-1). Substituent effects are discussed in terms of stability and compared with other similar compounds. The value of the standard molar enthalpy of formation for 3'-methylacetophenone was estimated from isomerization schemes. (C) 2012 Elsevier Ltd. All rights reserved.