Pseudomonas Fe-superoxide dismutase (superoxide:superoxide oxidoreductase, EC 1.15.1.1) is inactivated by hydrogen peroxide by a mechanism which exhibits saturation kinetics. The pseudo-first-order rate constant of the inactivation increased with increasing pH, with an inflection point around pH 8.5. Two parameters of the inactivation were measured in the pH range 7.8 to 9.0; the total H2O2 concentration at which the enzyme is half-saturated (K inact) was found to be independent of pH (30 mM) and the maximum rate constant for inactivation (k max) increased progressively with increasing pH, from 3.3 min-1 at pH 7.8 to 21 min-1 at pH 9.0. This evidence suggests the presence of an ionization group (pKa approximately 8.5) which does not participate in the binding of H2O2 but which affects the maximum inactivation rate of the enzyme. The loss of dismutase activity of the Fe-superoxide dismutase is accompanied by a modification of 1.6, 1.1 and 0.9 residues of tryptophan, histidine and cysteine, respectively. Since the amino acid residues of the Cr-substituted enzyme, which has no enzymatic activity, were not modified by H2O2, the active iron of the enzyme is essential for the modification of the amino acid residues.