Materials selection in the oil and gas industry relies on engineering standards, such as NACE TM0177 and NACE TM0284, which stipulate that oxygen pollution should be avoided during materials testing in H2S-containing media. In this paper, we explore the manner in which traces of oxygen can modify the test solution chemistry and the corrosion of/hydrogen permeation across iron membranes in H2S-containing solutions. Oxygen pollution is shown to strongly influence solution chemistry, through the introduction of sulfur-oxygen reaction products resulting in bulk acidification. Weight loss, electrochemical methods, and solution chemistry measurements conclude that iron corrosion rates in the presence of oxygen pollution are doubled, when compared against the control system (without oxygen pollution). Unexpectedly, despite a lower pH and higher corrosion rates in the oxygen-polluted H2S-containing solutions, the hydrogen permeation rate decreases monotonically, relative to the control. We discuss how this observation is most likely related to a disruption of sulfur adsorbates involved in hydrogen entry promotion.