The nonenzymatic reaction of glyoxylate and H202 was measured under physiological conditions of the pH and concentrations of reactants. The reaction of glyoxylate and H202 was secondorder, with a rate constant of 2.271 tool-1 s-1 at pH 8.0 and 25 ~ C. The rate constant increased by 4.4 times in the presence of Zn 2 + and doubled at 35 ~ C. We propose a mechanism for the reaction between glyoxylate and HzO 2 . From a comparison of the rates of H202 decomposition by catalase and the reaction with glyoxylate, we conclude that H202 produced during glycolate oxidation in peroxisomes is decomposed by catalase but not by the reaction with glyoxylate, and that photorespiratory CO2 originates from glycine, but not from glyoxylate, in C 3 plants. Simulation using the above rate constant and reported kinetic parameters leads to the same conclusion, and also makes it clear that alanine is a satisfactory amino donor in the conversion of glyoxylate to glycine. Some serine might be decomposed to give glycine and methylene-tetrahydrofolate; the latter is ultimately oxidized to CO 2 . In the simulation of the glycolate pathway of Euglena, the rate constant was high enough to ensure the decarboxylation of glyoxylate by H202 to produce photorespiratory CO 2 during the glycolate metabolism of this organism.