Cytochrome c peroxidase, freshly prepared, contains a penta-coordinated heme iron and is fully reactive with hydroperoxides. On the other hand, the enzyme normally stored in frozen states invariably contains different amounts of an altered, aged species whose heme iron is hexa-coordinated. The aged enzyme reacts with hydroperoxides only after a slow conformation change leading to the formation of a reactive penta-coordinated state. Thus, the reactivity of cytochrome c peroxidase with hydroperoxides is strongly controlled by the coordination state of the heme iron. A penta-coordinated heme iron may be a prerequisite for rapid reactions of hydroperoxidases with hydroperoxides.