The mean values of the standard massic energy of combustion of crystalline anhydrous alpha,alpha-trehalose (C12H22O11, polymorph beta) and crystalline dihydrate alpha,alpha-trehalose (C12H26O13) measured by static-bomb combustion calorimetry in oxygen, at the temperature T = 298.15 K, are Delta(c)u degrees = -(16434.05 +/- 4.50) J (.) g(-1) and Delta(c)u degrees = -(14816.05 +/- 3.52) J (.) g(-1), respectively. The standard (p degrees = 0.1 MPa) molar enthalpy of formation of these compounds were derived from the corresponding standard molar enthalpies of combustion, respectively, Delta H-f degrees(m) (C12H22O11,cr) = -(2240.9 +/- 3.9) kJ (.) mol(-1), and Delta H-f degrees(m) (C12H26O13, cr)= -(2832.6 +/- 3.6) kJ (.) mol(-1). The values of the standard enthalpies of formation obtained in this work, together with data on enthalpies of solution at infinite dilution (Delta H-sol(infinity)) for crystalline dihydrate and amorphous anhydrous trehalose, allow a better insight on the thermodynamic description of the trehalose system which can provide, together with the future research on the subject, a contribution for understanding the metabolism in several organisms, as well as the phase transition between the different polymorphs. (c) 2005 Elsevier Ltd. All rights reserved.