The standard (p degrees = 0.1 MPa) molar enthalpies of formation in the crystalline state of hydroxyapatite, chlorapatite and a preliminary value for bromapatite, at T = 298.15 K, were determined by reaction-solution calorimetry as: Delta(f)H(m)degrees[Ca-10(PO4)(6)Br-2, cr] = (OH)(2), cr] = -(13399 +/-11) kJ(.)mol(-1), Delta(f)H(m)degrees[Ca-10(PO4)(6)Cl-2, cr] = -(13231 +/- 82) kJ(.)mol(-1), and Delta(f)H(m)degrees[Ca-10(PO4)(6)Br-2, cr] = -(13063 +/- 81) kJ(.)mol(-1). A critical assessment of these results and of previously published data is made. Finally, the standard molar enthalpy of formation of iodapatite is estimated as Delta(f)H(m)degrees[Ca-10(PO4)(6)I-2, cr] = -12949 kJ(.)mol(-1), from a linear correlation of Delta(f)H(m)degrees[Ca-10(PO4)(6)X-2, cr] (X = OH, F, Cl, Br) against the corresponding Delta(f)H(m)degrees[CaX2, cr]. (c) 2005 Elsevier Ltd. All rights reserved.