The coordination of alkyl peroxides by heme proteins is an important step in the functioning of several enzymes, (for example, peroxidases, cytochrome P450, estrogen synthase). Generally the reaction of peroxides with iron porphyrins in the absence of a proton results in the destruction of both porphyrin and peroxide. However, three distinct intermediates involving coordination of peroxides to iron porphyrins can be identified using spectroscopic techniques. The formation, spectroscopic characteristics and chemical behavior of these reactive intermediates are reviewed here.