The temperature dependence of the dissociation of cyanide (CN-) from cyanomyoglobin (Mb(II)CN), the cyanide complex of myoglobin, has been studied by cyclic voltammetry. Direct electrochemical reduction of cyanometmyoglobin initiates the dissociation of the cyanide ligand. The temperature dependence of the heterogeneous electron transfer kinetics, the kinetics of cyanide dissociation, and the formal potential for Mb(III)CN have been determined over the temperature range of 10-50 degrees C. The latter information has been used to determine the reaction center entropy change (Delta S-rc) of this reaction to be - 90 +/- 5 J mol(-1) K-1. The water complex of myoglobin, aquometmyoglobin (Mb(III)H2O), was also studied and the reaction center entropy for this system was determined to be - 40 +/- 4 J mol(-1) K-1. Differential scanning calorimetry was also used to determine the transition temperature and enthalpy changes for each component of these reactions. The results given here provide an indirect model for the in vivo reaction of the oxymyoglobin complex (Mb(II)O-2), that is, the dissociation of dioxygen from the reduced form of myoglobin.