An experimental study on solid-state stability of a detergent protease (Savinase®) is reported. The inactivation kinetics of technical grade enzyme powder was determined as a function of gas phase H2O2 concentration and humidity by employing a quick assay running over few hours instead of several weeks as typical in industry. The results indicated that enzymes adsorbed significant amounts of moisture and H2O2 during exposure. The amount of adsorbed H2O2 did not depend on humidity in the gas stream, which implied that water and H2O2 were not competing for the same adsorption sites. Inactivation of the solid-state enzyme was caused by the mutual effect of increasing hydration and H2O2 (g) concentration. No auto-proteolytic activity or covalently bound aggregate formation was detected. A simple mechanism for solid-state enzyme oxidation was proposed and the kinetic parameters in the resulting rate expression of inactivation were derived. A good agreement between the derived equation and experimental data was obtained. The oxidative alterations on Savinase® were investigated by peptide mapping. Molecular mass examination of CNBr-cleaved fragments by MALDI-TOF mass spectroscopy located the oxidation-labile residue. Only one methionine (Met 222) was oxidized, while other residues remained unaffected. The study provides practical information on solid-state stability measurements of biocatalysts in oxidative environments.