An NMR double resonance study of a mixture of ferricytochrome c and azidoferricytochrome c reveals the presence of an exchange of azide ions between the protein molecules. Irradiation of one of the several resolved methyl resonances from ferricytochrome c cross saturates a correspondingly unique resonance of azidoferricytochrome c and vice versa. The hyperfine shifted resonances of azidoferricytochrome c are thus correlated with those of ferricytochrome c and assigned to porphyrin ring methyls. "On-Off" rates for the azide ion are determined. © 1970.