The binding of carbon monoxide to ferrous chloroperoxidase in the pH range 4-6.5 is influenced by a titratable group on the enzyme having a pKA of 5.5 +/- 0.2 at 20 degrees C. The basic form of the enzyme reacts much faster with carbon monoxide than does the protonated form of the enzyme. The delta H degrees for the ionization of the functional group in the enzyme involved in carbon monoxide binding is about 8 kcal mol-1, and the delta S degrees is approximately 1 cal mol-1 K-1. These pKA and delta H degrees values suggest that this functional group is an imidazole ring associated with a histidine residue situated at the active site of the enzyme. The rates of the reaction for the formation and dissociation of the complex suggest that this histidine residue is not directly liganded to the iron atom of the heme prosthetic group. The relatively good agreement between the various kinetic approaches with several methods of experimentation, data collection, and data analysis lends strength to a proposed model in which the histidine occupies a distal site close to the sixth axial ligand position of the heme iron atom.