In efforts to probe the function of the O-2-reduction site of bovine heart resting oxidized cytochrome c oxidase, highly sensitive resonance Raman analyses of azide binding to the O-2-reduction site show that azide is simultaneously accessible to Fe and Cu ions in the O-2-reduction site. This indicates that the site has spatial multiplicity for accepting significantly larger external ligands than expected from its X-ray structure. This could contribute to the accurate coupling between the O-2 reduction and the proton pump.