The standard (p degrees = 0.1 MPa) molar energies of combustion, Delta U-c(m)degrees for the crystalline 2-thiophenecarboxylic acid hydrazide and 2-furancarboxylic acid hydrazide were determined, at the temperature of 298.15 K, by rotating bomb and static bomb combustion calorimetry, respectively. For these compounds, the standard molar enthalpies of sublimation, Delta H-g(cr)m degrees at T = 298.15 K, were derived by the Clausius-Clapeyron equation, from the temperature dependence of the vapour pressures of these compounds, measured by the Knudsen effusion mass-loss technique. The results were as follows: [GRAPHICS] These values were used to derive the standard molar enthalpies of formation of the title compounds in their gaseous phases and the results are discussed in terms of energetic effects of the introduction of the -CONHNH2 group in the thiophene and furan rings. Using estimated values for the heat capacity differences between the gas and the crystal phases of the studied compounds, the standard (p degrees = 0.1 MPa) molar enthalpies, entropies, and Gibbs energies of sublimation, at T = 298.15 K, were derived. (C) 2008 Elsevier Ltd. All rights reserved.