The equilibrium reactions: 4CaKPO(4)(s) + 6Ni(s) reversible arrow CaK4(PO4)(2)(s) + 3CaO + 2Ni(3)P(s) + 5/2O(2)(g), 4CaKPO(4)(s) + 3K(4)P(2)O(7)(s) + 6Ni(s) reversible arrow 4CaK(4)(PO4)(2)(s) + 2Ni(3)P(s) + 5/2O(2)(g), 4CaK(2)P(2)O(7)(s) + 6Ni(s) reversible arrow 4CaKPO(4)(s) + K4P2O7(s) + 2Ni(3)P(s) + 5/2O(2)(g) and Ca10K(PO4)(7)(s) + 9CaK(2)P(2)O(7)(s) + 18Ni(s) reversible arrow 19CaKPO(4)(s) + 6Ni(3)P(s) + 15/2O(2)(g) were studied in the temperature range from 880 to 1125 K. The oxygen equilibrium pressures were determined using galvanic cells incorporating yttria stabilised zirconia as solid electrolyte. From the measured data and using literature values of standard Gibbs free energy of formation for CaO, Ni3P and K4P2O7, the following relationship of the standard Gibbs free energy of formation for CaKPO4, CaK4(PO4)(2), CaK2P2O7, and Ca10K(PO4)(7) were calculated: Delta(f)G(circle)(CaKPO4) +/- 5.2/(kJ.mol(-1)) = - 1506.6 - 3.2933(T/K) + 0.4603(T/K) . ln(T/K) 890 < (T/K) < 1125 Delta(f)G(circle)(CaK4(PO4)(2)) +/- 8.9/(kJ mol(-1)) = -3498.2 -2.1339(T/K) + 0.3661(T/K) . ln(T/K) 930< (T/K) < 1125 Delta(f)G(circle)(CaK2P2O7) +/- 7.2/(kJ.mol(-1)) = -2690.0 -3.7820(T/K) + 0.5599(T/K) . ln(T/K) 940 < (T/K) < 1100 Delta(f)G(circle)(Ca10K(PO4)(7)) +/- 133/(kJ.mol(-1)) = -5988.3 -57.063(T/K) + 7.4407(T/K) . ln(T/K) 880 < (T/K) < 1015. (C) 2007 Elsevier Ltd. All rights reserved.