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The oxidation kinetics of glutathione (GSH) by hydrogen peroxide has been studied at neutral pH for different concentration ratios [GSH]0/[H2O2]0 between 0.2 and 2 (5 × 10−4 M ≤ [H2O2]0 ≤ 2.5 × 10−3M; 4 × 10−4 M ≤ [GSH]0 ≤ 2.5 × 10−3 M). In all cases studied, glutathione disulfide GSSG is the main product formed via two different oxidation ways, each of them contributing respectively to 80–85% and 10–15%.Our kinetic data indicate that an important fraction of hydrogen peroxide disappears without oxidizing the thiol function. This can be attributed to a combination between GSH and H2O2 protecting the sulfide group. Chemical evidences of the existence of a peroxide bond with GSSG are described. In our experimental conditions, the overall oxidation equation is 2mol GSH reacting with 2mol H2O2 giving 1mol GSSG. It is very different from the usually accepted stoichiometric reaction:[Formula: see text]A kinetic scheme is proposed and the corresponding rate constants are determined. Keywords: glutathione, hydrogen peroxide, kinetic mechanism, glutathione disulfide.