C-13 NMR signals of (CO)-C-13 free and bound to hemoglobin (Hb(13)CO) were investigated at 9.4 T. Existence of (CO)-C-13 exchange between alpha and beta hemoglobin irons was proven by C-13 NMR temperature line splitting and by the shift of the (CO)-C-13 NMR signal, after mixing alpha (CO)-C-13 with deliganded beta or beta (CO)-C-13 with deliganded alpha. C-13 NMR bandwidth of (CO)-C-13 bound to hemoglobin was investigated for various HbCO ligandation levels: from low (16%) to full (CO)-C-13 association, the kinetics of the (CO)-C-13 exchange decreases. A model using the exchange kinetics can generate the sigmoidal shape of the hemoglobin ligandation curve.