The ion currents of gaseous phosphorus oxides in equilibrium with a mixture of Al2O3 center dot P2O5 and Al2O3 at T = (1473 to 1573) K were measured by double Knudsen cell mass spectrometry. The Gibbs free energy changes of the following reactions estimated in comparison with the ion currents from a mixture of (MgO)(3)center dot P2O5 and MgO are as follows: Al2O3(s) + P-2(g) + 5/2O(2)(g) = Al2O3 center dot P2O5(s), 2 Delta G degrees/J = -1970000 + 574T/K(+/- 28000) for (1473 < T/K < 1573). (C) 2010 Elsevier Ltd. All rights reserved.