The mean values of the standard massic energy of combustion of caffeine in phase I (or alpha) and in phase II (or beta) measured by static-bomb combustion calorimetry in oxygen, at T = 298.15 K, are Delta(c)u degrees (C8H10O2N4, I) = -(21823.27 +/- 0.68) J center dot g(-1) and Delta(c)u degrees (C8H10O2N4, II) = -(21799.96 +/- 1.08) J center dot g(-1), respectively. The standard (p degrees = 0.1 MPa) molar enthalpy of formation in condensed phase for each form was derived from the corresponding standard molar enthalpies of combustion as, Delta H-f(m)degrees (C8H10O2N4, cr, I) = -(340.6 +/- 2.3) kJ center dot mol(-1) and Delta H-f(m)degrees (C8H10O2N4, cr, II) = -(345.1 +/- 2.3) kJ center dot mol(-1). The difference between the standard enthalpy of formation of the two polymorphs in condensed phase was also evaluated by using reaction-solution calorimetry. The obtained result, 2.04 +/- 0.25 kJ center dot mol(-1), is in agreement, within the uncertainty, with the difference between the molar enthalpies of formation obtained from combustion experiments (4.5 +/- 3.2) kJ center dot mol(-1), which can be considered as an internal test for consistency of the results. A value for the standard enthalpy of formation of caffeine in the gaseous state was proposed: Delta H-f(m)degrees (C8H10O2N4, g) = -(229.7 +/- 6.1) kJ center dot mol(-1), estimated from the values of the standard enthalpies of formation of both crystalline forms obtained in this work, and the data on standard enthalpies of sublimation collected from the literature. (c) 2006 Elsevier Ltd. All rights reserved.