The microstructure evolution and phase formation sequence has been examined during the initial oxidation exposure of Hf-Ta alloys at 1500 degrees C for several alloy compositions. The oxidation reaction path involves the conversion of the initial beta Hf-Ta BCC solid solution substrate phase into a two-phase monoclinic HfO2 and a Ta rich BCC solid solution reaction layer that undergoes a further reaction between HfO2 and Ta oxide to yield and outer Hf6Ta2O17 superstructure. The overall reactive diffusion pathway is consistent with the calculated Hf-Ta-O ternary phase diagram. The oxidation reaction rate exhibited a minimum at Hf 26.7 at.%Ta where the outer oxide was single phase Hf6Ta2O17. For other compositions the outer oxide was composed of Hf6Ta2O17 and either HfO2 (Hf-rich compositions) or Ta2O5 (Ta-rich compositions). The enhanced oxidation resistance for the Hf-26.7 at.%Ta alloy is attributed to the adherent Hf6Ta2O17 superstructure. (C) 2017 Elsevier B.V. All rights reserved.