The standard (p degrees = 0.1 MPa) molar enthalpies of formation, in the crystalline state, of the three thiophenecarboxylic acid derivatives, at T = 298.15 K, were derived from the standard massic energies of combustion, measured by rotating-bomb combustion calorimetry. The Knudsen mass loss effusion method was applied to determine the temperature-vapour pressure dependence, from which the standard molar enthalpies of sublimation, at T = 298.15 K, were derived by fitting data to the Clausius-Clapeyron equation. The values obtained were: [GRAPHICS] The standard (p degrees = 0.1 MPa) molar enthalpies of formation, in the gaseous phase, were derived from the experimental results and the enthalpy of isomerisation between 3-methyl- and 5-methyl-2-thiophenenecarboxylic acids was calculated. Moreover, the results are compared with the analogue benzene derivatives and discussed in terms of molecular structure. (C) 2008 Elsevier Ltd. All rights reserved.