The present work reports an experimental thermochemical study supported by state of the art calculations of two heterocyclic
compounds containing oxygen in the ring: xanthone and tetrahydro-γ-pyrone. The standard (pº = 0.1 MPa) molar enthalpies of formation in the condensed phase, at T = 298.15 K, were derived from the measurements of the standard molar energies of combustion in oxygen atmosphere, using a
static bomb calorimeter. The standard molar enthalpies of sublimation or vaporization, at T = 298.15 K, of the title compounds were obtained from Calvet microcalorimetry measurements. These values were used to derive
the standard enthalpies of formation of the compounds in the gas-phase at the same temperature, which were compared with estimated
data from G3(MP2)//B3LYP computations.