We have studied absorption spectral changes and heme modifications in the reaction of horseradish peroxidase (HR-P), myoglobin (Mb), or bovine liver catalase (BLC) with hypochlorous acid (HOCl). Under acidic condition, HOCl is easily released from an Fe-III-HOCl complex. In contrast, the O-Cl bond cleavage in an Fe-III-OCl intermediate occurs under neutral and alkaline conditions. The heterolysis proceeds in HR-P and BLC, but the homolysis in Mb seems to be associated with the generation of meso-chlorinated heme adduct.