Equilibrium binding experiments have been performed with perchlorate, chloride, and acetate in the presence of horseradish peroxidase. The binding of perchlorate and acetate appears to be like that of nitrate, at a site other than the sixth coordination position of the heme iron. Competitive experiments using both nitrate and cyanide demonstrate that two different binding sites are present on the enzyme. Chloride appears to bind at the sixth coordination position as do both fluoride and cyanide. Temperature jump experiments indicate that it is likely the nitrate anion and not undissociated nitric acid which is the binding species. Competitive stopped flow experiments indicate that the bound nitrate slows both the association rate and dissociation rate of cyanide, indicating that nitrate binds close to the sixth coordination position.