Addition of hydrogen peroxide (H₂O₂) is a method commonly used to trigger cellular oxidative stress. However, the doses used (often hundreds of micromolar) are disproportionally high with regard to physiological oxygen concentration (low micromolar). In this study using polarographic measurement of oxygen concentration in cellular suspensions we show that H₂O₂ addition results in O₂ release as expected from catalase reaction. This reaction is fast enough to, within seconds, decrease drastically H₂O₂ concentration and to annihilate it within a few minutes. Firstly, this is likely to explain why recording of oxidative damage requires the high concentrations found in the literature. Secondly, it illustrates the potency of intracellular antioxidant (H₂O₂) defense. Thirdly, it complicates the interpretation of experiments as subsequent observations might result from high/transient H₂O₂ exposure and/or from the diverse possible consequences of the O₂ release.