Solutions consisting of 2:1 mole excess of yeast isozyme-1 (iso-1) ferricytochrome c in combination with either resting-state cytochrome c peroxidase (CcP) or cyanide-ligated cytochrome c peroxidase (CcPCN) consist of equimolar concentrations of the noncovalent 1:1 complex (peroxidase/ferricytochrome c) and ferricytochrome c free in solution. This work reveals that in these solutions the ferricytochrome c is in dynamic exchange between the peroxidase bound (b) environment and the free (f) environment. The exchange is in the slow-intermediate regime on the NMR time scale because solutions of these mixtures simultaneously display 0n iso-1 ferricytochrome c heme 3-CH3 resonance from ferricytochrome c molecules in both free and bound environments. Two types of magnetization-transfer experiments have been used to measure the preexchange lifetimes (tau) of iso-l ferricytochrome c in the free and bound environments : saturation transfer and inversion transfer. The results show that tau(b) and tau(f) range between 1 and 4 ms, depending upon temperature and concentration. The temperature dependence is not great. Arrhenius graphs show that the activation energy for exchange is 7 +/- 2 kcal/mol. Furthermore, the concentration dependence of the lifetime indicates that the exchange process is not governed by unimolecular dissociation and that tau is related to a bimolecular rate constant. These results imply that a peroxidase mechanism must incorporate concepts such as (i) "substrate assisted" product dissociation from the intermediate electron-transfer complexes or (ii) two cytochrome c binding sites on CcP.