NADPH is known to be tightly bound to mammalian catalase and to offset the ability of the substrate of catalase (H2O2) to convert the enzyme to an inactive state (compound II). In the process, the bound NADPH becomes NADP+ and is replaced by another molecule of NADPH. This protection is believed to occur through electron tunneling between NADPH on the surface of the catalase and the heme group within the enzyme. The present study provided additional support for the concept of an intermediate state of catalase, through which NADPH serves to prevent the formation (rather than increase the removal) of compound II. In contrast, the superoxide radical seemed to bypass the intermediate state since NADPH had very little ability to prevent the superoxide radical from converting catalase to compound II. Moreover, the rate of NADPH oxidation was several times the rate of compound II formation (in the absence of NADPH) under a variety of conditions. Very little NADPH oxidation occurred when NADPH was exposed to catalase, H2O2, or the superoxide radical separately. That the ratio exceeds 1 suggests that NADPH may protect catalase from oxidative damage through actions broader than merely preventing the formation of compound II.