The energetic study of benzoxazole, 5-chloro-2-methylbenzoxazole, and 2-chlorobenzoxazole, in condensed and gaseous states, has been developed using experimental techniques and computational approaches. The values of the standard (p degrees = 0.1 MPa) molar enthalpy of formation, at T = 298.15 K, of crystalline benzoxazole (36.0 +/- 2.0) kJ . mol(-1) and 5-chloro-2-methylbenzoxazole (145.6 +/- 2.2) kJ . mol(-1) and liquid 2-chlorobenzoxazole (52.5 +/- 3.0) kJ . mol(-1) were determined from the corresponding experimental standard molar energy of combustion in oxygen, -(3432.1 +/- 1.7) kJ . mol(-1), -(3883.0 2.0) kJ . mol(-1), and -(3298.0 +/- 2.8) kJ . mol(-1), respectively, measured by static or rotating-bomb combustion calorimetry. At T = 298.15 K, the standard (p degrees = 0.1 MPa) molar enthalpy of sublimation of benzoxazole and 5-chloro-2-methylbenzoxazole and of vaporization of 2-chlorobenzoxazole, (69.2 +/- 0.8) kJ . mol(-1), (82.4 +/- 2.2) kJ . mol(-1), and (56.3 +/- 1.6) kJ . mol(-1) respectively, were determined by a direct method, using the vacuum drop microcalorimetric technique. From the latter values and from the enthalpies of formation of the condensed compounds, the standard (p degrees = 0.1 MPa) enthalpies of formation of the gaseous compounds have been calculated. Standard ab initio molecular orbital calculations were performed using the G3(MP2)//B3LYP composite procedure and several working reactions in order to derive the standard molar enthalpy of formation of the three compounds. There exists a good agreement between the experimental and the computational data. (C) 2012 Elsevier Ltd. All rights reserved.