The equilibrium reactions: 3Ca(2)P(2)O(7)(s) + 6Ni(s) reversible arrow 2Ca(3)(PO4)(2)(s) + 2Ni(3)P(s) + 5/O-2(g) and 2Ca(PO3)(2)(s) + 6Ni(s) reversible arrow Ca2P2O7(s) + 2Ni(3)P(s) +/- 5/2O(2)(g) were studied in the temperature range 890 K to 1140 K. The oxygen equilibrium pressures were determined using galvanic cells incorporating yttria stabilized zirconia as solid electrolyte. From the measured data and using the literature values of standard Gibbs free energy of formation for Ca-3(PO4)(2) and Ni3P, the following relationship of the standard Gibbs free energy of formation for Ca2P2O7 and Ca(PO3)(2) were calculated: A(f)G degrees(Ca2P2O7) +/- 11/(kJ center dot mol(-1)) -3475.9 + 1.5441 (T/K) -0.1051 (T/K) center dot 1n(T/K) and sA(f)G degrees(Ca(PO3)(2)) +/- 12/(kJ center dot mol(-1)) -3334.8 + 6.1561 (T/K) -0.6950(T/K) center dot 1n(T/K). (c) 2006 Elsevier Ltd. All rights reserved.