The vapor pressure of phosphorus in equilibrium with a mixture of elemental Cr and Cr3P at (1523 to 1623) K was estimated by double Knudsen cell mass spectrometry with a Cu-P alloy as a reference substance. The Gibbs free energy of formation of Cr3P was calculated from the vapor pressure as follows: 3Cr(s) + 1/2P(2)(g) = Cr3P(s) Delta(f)G degrees = -261,000 + 83.0(T/K) +/- 6800 J . mol(-1) (1523 < T/K < 1623) (c) 2007 Elsevier Ltd. All rights reserved.