The standard (p degrees = 0.1 MPa) molar enthalpies of combustion of 1-(2H)-phthalazinone and phthalhydrazide, both in the solid phase, were measured at T = 298.15 K by static bomb calorimetry. Further, the standard molar enthalpies of sublimation, at T = 298.15 K, of these two phthalazine derivatives were derived from the Knudsen effusion technique. The combustion calorimetry results together with those obtained from the Knudsen effusion technique, were used to derive the standard molar enthalpies of formation, at T = 298.15 K, in the gaseous phase for 1-(2H)-phthalazinone and phthalhydrazide, respectively as, (79.1 +/- 1.8) kJ . mol(-1) and -(107.4 +/- 2.4) kJ . mol(-1). (c) 2008 Elsevier Ltd. All rights reserved.