The production of H2O2 by isolated mitochondria is frequently used as a measure of mitochondrial superoxide formation. Matrix superoxide dismu-tase quantitatively converts matrix superoxide to H2O2. However, matrix enzymes such as the glutathione peroxidases can consume H2O2 and com-pete with efflux of H2O2 , causing an underestimation of superoxide produc-tion.