Values of the standard of (p(o) = 0.1 MPa) molar enthalpy of combustion, Delta(c)H(m)(o), of four crystalline compounds: 5-aminouracil, 6-aminouracil, 6-amino-1-methyluracil, and 6-amino-1,dimethyluracil, were determined, at T = 298.15 K. using a static bomb combustion calorimeter. The values obtained of standard molar enthalpy of combustion were used to derive the standard molar enthalpy of formation of the compounds investigated in their condensed phase and together with literature values of the standard molar enthalpy of sublimation, yielded the standard molar enthalpies of formation in the gaseous phase. These are discussed in terms of the effects of the molecular structure on the relative enthalpic stability. (C) 2011 Elsevier Ltd. All rights reserved.