The effect of O on the phase relations at 950 degrees C in Fe-15Cr-1Nb alloys is experimentally investigated. Fe-15Cr-1Nb alloys are oxygenated by subjecting high-purity Fe-15Cr-1Nb to an 0 atmosphere at 600 degrees C. Both the high-purity and the oxygenated Fe-15Cr-1Nb alloys are heat treated for up to 500 h at 950 degrees C, quenched and investigated by scanning electron microscopy, transmission electron microscopy and electron probe microanalysis. The results show that Fe2Nb is in equilibrium with alpha (Fe, Cr) with 0.29 at.% Nb in solid solution in the pure Fe-15Cr-1Nb alloy. The presence of a small amount of O induces the precipitation of a Fe6Nb6Ox phase with a cubic crystal structure and lattice parameter 1.13 nm, thereby decreasing the Nb in solid solution in alpha (Fe, Cr) with increasing O content. (C) 2010 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.