The reversibility of catalase inhibition was studied for a number of substances. The action of catalase in the presence of the inhibitors was followed manometrically. The reversibility of the inhibition was studied in specially made manometer flasks in which the concentration of the inhibitor could be decreased without any other variation. All experiments were carried out in a phosphate buffer solution of pH 7.4 using purified horse liver as the substrate. The effects of the following on catalase oxygen production were examined: hydroxylamine-hydrochloride (5470111) at concentrations of 0, 0.00001 Moles (M), 0.0001M, and 0.001M; hydrazine-sulphate (10034932) at 0, 0.01M, 0.1M; phenylhydrazine-hydrochloride (59881) at 0, 0.0001M, 0.001M, 0.01M, and 0.1M; sodium-azide (26628228) at 0, 0.000001M, 0.00001M, and 0.0001M; potassium-chlorate (3811049) at 0, 0.0001M, 0.001M, and 0.01M; mercuric-chloride (7487947) at 0, 0.00001M, 0.0001M, and 0.001M; resorcinol (108463) at 0, 0.0001M, 0.001M, 0.01M, and 0.1M; benzidine (92875) at 0, 0.001M, and 0.01M; p-phenylenediamine (106503) at 0, 0.01M, and 0.1M; and m-phenylenediamine (108452) at 0, .01M, and 0.1M. Hydroxylamine-hydrochloride, hydrazine-sulphate, phenylhydrazine-hydrochloride, sodium-azide, resorcinol, p-phenylenediamine, and m-phenylenediamine gave reversible inhibitions. The mercuric-chloride inhibition was partly reversible. Potassium-chlorate gave an irreversible inhibition. The author notes that all substances known to yield methemoglobin compounds act as reversible inhibitors of catalase. This suggests that catalase may be a hemin compound. Mercuric-chloride may act on the non hemin part of the enzyme molecule.