The research objective was to determine the effect of aluminum alloy additions on the oxidation behavior of cobalt-nickel alloys. Thirty high-purity alloys were reacted with 02 at 0.5 Atm at 1,351 deg, 1,389 deg, and 1,429 deg k in a volumetric apparatus for about 300 min. The alloy compositions consisted of co-ni master alloys (0, 22, 50, 70, 91, and 100 wt pct ni) with aluminum additions of about 0, 1, 2, 4, and 8 wt pct. Large early deviations from, or changes in, the parabolic rate constant were encountered, but in most cases, the reaction rate declined approximately in accordance with the parabolic rate law. A sharp peak in the parabolic oxidation rate constant was observed for specimens containing 0 to 22 percent nickel and 1 to 2 percent aluminum at 1,351 deg k, and the rate constants for these compositions were lower at the higher temperatures. Specimens containing 8 percent aluminum reacted far more slowly than did any of the others.