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