In the present work, a combined experimental and computational thermochemical study of 2-(5H)-furanone and 2-(5H)-thiophenone was carried out. The standard (p degrees = 0.1 MPa) molar enthalpies of formation, in the liquid phase, for the 2-(5H)-furanone and 2-(5H)-thiophenone, at T=298.15K, were derived from the standard massic energies of combustion, measured by static and rotating bomb combustion calorimetry, respectively. High temperature Calvet microcalorimetry experiments were performed to measure their standard molar enthalpies of vaporization. [GRAPHICS] These experiments allowed the determination of the standard molar enthalpies of formation, in the gaseous phase, at T = 298.15 K. These values are compared with estimated data from G3(MP2)//B3LYP Computations, using several gas-phase working reactions. (C) 2009 Elsevier Ltd. All rights reserved.