The standard (p degrees = 0. 1 MPa) molar enthalpies of formation, Delta(f)H degrees(m), for crystalline 1 -hydroxyisoquinoline, 5-hydroxyisoquinoline and 1,5-diidroxyisoquinoline, were derived from the standard molar enthalpies of combustion, in oxygen, at the temperature 298.15 K, measured by static bomb-combustion calorimetry. The standard molar enthalpies of sublimation, Delta(cr)(g)H degrees(m), at T = 298.15 K, were determined by Calvet microcalorimetry. The results were as follows: [GRAPHICS] The derived standard molar enthalpies of formation, in the gaseous state, are analysed in terms of enthalpic increments and interpreted in terms of molecular structure. (c) 2005 Elsevier Ltd. All rights reserved.