Thermodynamic properties of 9,9-dimethylxanthene and 2,7-di-tert-butyl-9,9-dimethylxanthene for the condensed and gas states were derived from experimental and computational studies. Static-bomb combustion calorimetry, vacuum drop microcalorimetry and the Knudsen effusion techniques were used. Computational calculations of the enthalpies of hypothetical reactions in the gaseous phase, using the G3(MP2)//B3LYP composite method, were performed for the two xanthene derivatives. Natural bond orbital (NBO) calculations were also performed to ascertain the structure and reactivity of these compounds. The energetic effects caused by replacing hydrogen atoms in the xanthene moiety by methyl and tertbutyl groups yielding 9,9-dimethylxanthene and 2,7-di-tert-butyl-9,9-dimethylxanthene species were determined from direct comparison of their standard (p degrees = 0.1 MPa) molar enthalpies of formation in the gaseous phase, at T=298.15 K. (C) 2016 Elsevier Ltd.