Mildly acidic water containing dissolved H2S presents a strong risk in the cracking of low-carbon steels. Several studies on H2S cracking mechanisms have shown that the main driving force is linked to the ability of H2S to promote hydrogen entry into the bulk material. Standard test methods have been developed and published as NACE technical standards (e.g. NACE TM0284 and NACE TM0177) to aid materials selection in the oil and gas sector. Though it is recognized that oxygen pollution should be avoided during H2S cracking tests, there is a lack of experimental data to illustrate the effects of a small oxygen pollution. Dissolved oxygen concentrations greater than the recommended upper limit (50 parts per billion) can easily be obtained in the case of poor laboratory practices. This paper will focus on the interactions between oxygen and H2S on electrochemical behavior of unalloyed steel. A continuous O2 injection at a level corresponding to 500 ppb is applied, together with H2S bubbling in our test solutions, for periods lasting the same order as SSC standard tests. Steel surface reaction phenomena/corrosion rates in H2S saturated solution, with or without oxygen pollution, are studied using electrochemical impedance spectroscopy. The evolution of corrosion rates obtained from impedance analysis was compared to two other independent methods: i/ weight loss measurements and, ii/ hydrogen permeation. Without O2 pollution, a permeation efficiency of 100% was obtained, as expected. Permeation current density was thus found to match precisely with the corrosion current density determined by impedance analysis at different times. On the other hand, when a continuous O2 pollution was added in the system, significantly higher corrosion rates were observed, associated with test solution acidification. At the same time, permeation efficiency was decreased by up to one order of magnitude.