As part of a development effort on nickel aluminides based on Ni3Al as structural materials for fossil energy applications, oxidation/sulfidation studies are being conducted at Argonne National Laboratory on materials that are being developed at Oak Ridge National Laboratory. Sheet samples of nickel aluminide, containing 23.5 at, % Al, 0.5 at. % Hf, and 0.2 at. % B were tested in an annealed condition and after preoxidation treatments. Continuous weight-change measurements were made at 875°C by a thermogravimetric technique in exposure atmospheres of air, a low-p02 gas mixture, and low-p02 gas mixtures with several levels of sulfur. getailed analyses of the corhosion product scale layers were conducted using a scanning electron microscope equipped with an energy dispersive x-ray analyzer and an electron microprobe. The air-exposed specimens developed predominantly nickel oxide; the specimen exposed to a low-p02 environment developed an aluminum oxide scale. As the sulfur content of the gas mixture increased, the alumina scale exhibited spallation and the alloy tended to form nickel sulfide as the reaction phase. The results indicated that the sulfidation reaction in nickel aluminide specimens (both bare and preoxidized) was determined by the rate of transport of nickel from the substrate through the scale to the gas/alumina scale interface, mechanical integrity of the scale, and the H2S concentration in the exposure environment.