The present work examines the influence trace levels of air contamination (O2, H2O, etc.) have on the oxidation behavior of IN718 at elevated temperatures. IN718 specimens were heated to 1050 °C for 2 h in a TG equipped with three separate gas inputs: Pure He, He-0.219%Air (460 ppm O2)), and He-H2O (300 ppm) mixtures. The samples were characterized using TGA (mass gain/loss), GCMS (Contaminant pickup), and SEM-EDS (oxide morphology, thickness, and composition). Oxidation was shown to be governed by the atmospheric O2 and H2O content, on a mol-to-mol basis. The formation of discernable Cr2O3 and TiNbOx layers required a total O2 and H2O content of >100 ppm. Severe spalling of the Cr2O3 occurred at the highest concentrations of O2 assessed while the Cr2O3 layers formed as a result of H2O were much more adherent. The parabolic rate calculated from the TG results demonstrated a clear correlation between the oxide growth rate and contaminant concentration, which plateaued at the higher concentration, thus inferring rate limiting diffusion of oxide formers (Cr, Ti, Nb) with O2 and H2O, as the surface became depleted.