The instability of pyruvate in neutralized perchloric acid extracts of erythrocytes is well-known from the literature. As shown in this study, the main reason for this phenomenon is a reaction of pyruvate with hydrogen peroxide, which is formed from oxyhemoglobin during acid precipitation. Thereby, pyruvate is oxidized to carbon dioxide and acetate. This decomposition is easily prevented by addition of catalase to the neutralized extracts, whereby authentic pyruvate can be determined precisely. Similarly, GSH and NAD(P)H oxidation in the extracts is prevented by addition of catalase.