Reaction of H2O2 with iron containing superoxide dismutase (SOD) was studied by absorption spectroscopy, activity measurement and amino acid analysis. The enzyme showed new absorption peaks at 255 and 310 nm by the reaction with H2O2. The time course of the absorbance change at 320 nm was the same as that of the enzyme inactivation in the initial period of time. The amino acid analysis and the colorimetric determination of the H2O2 treated enzyme showed the destruction of 1.5-1.8 tryptophan residues per dimer of the enzyme.