A novel 'thermodynamic' mechanistic rationale of haloperoxidase catalysis is based on the following two assumptions: (i) the role of enzyme consists only in the rapid equilibration between the halogen-containing species originating from halide and hydrogen peroxide; (ii) the interaction between the enzyme and organic substrate is kinetically insignificant and halogenation occurs as a result of the electrophilic attack of the active brominating (Br3-, Br2 and HBrO) or chlorinating (HCIO) species at monochlorodimedon indicative of a higher chloride 'specificity' of chloroperoxidase from C. fumago.