The standard ( p o = 0.1 MPa) molar enthalpies of combustion in oxygen, at T = 298.15 K, for crystalline 2-iodosobenzoic acid, (OI)C 6 H 4 COOH, and 4-nitrosophenol, (ON)C 6 H 4 OH, were measured by rotating-bomb calorimetry and static-bomb calorimetry, respectively. These values were used to derive the standard molar enthalpies of formation of the crystalline compounds.