A thermophysical and thermochemical study has been carried out for crystalline imidazolidin-2-one and N,N'-trimethyleneurea [tetrahydropyrimidin-2(1H)-one]. The thermophysical study was made by differential scanning calorimetry, d.s.c., in the temperature intervals between T = 268 K and their respective melting temperatures. Several solid-solid transitions have been detected in imidazolidin-2-one. The standard (p degrees = 0.1 M Pa) molar enthalpies of formation, at T = 298.15 K, for crystalline imidazolidin-2-one and N,N'-trimethyleneurea [tetrahydropyrimidin-2(l H)-one], were deter-mined using static-bomb combustion calorimetry. The standard molar enthalpies of sublimation, at T = 298.15 K, for the two compounds were derived from the variation of their vapour pressures, measured by the Knudsen effusion method, with the temperature. These two thermochemical parameters yielded the standard molar enthalpies of formation of the two cyclic urea compounds studied in the gaseous phase at T = 298.15 K. These values are discussed in terms of molecular structural contributions and interpreted on the bases of the "benzo-condensed effect" and of the ring strain of imidazolidin-2-one. (c) 2007 Elsevier Ltd. All rights reserved.