Electrochemistry and spectroelectrochemistry were used to study solutions of horse myoglobin, both by itself and complexed with cyanide or imidazole, at indium-tin oxide electrodes. In the absence of ligands other than water, myoglobin exhibited slow quasi-reversible heterogeneous electron transfer kinetics, with a formal heterogeneous electron transfer rate constant (k(o＇)) value of 1.6(+/- 0.4) x 10(-5) cm s(-1). Differences from simple simulations in the reverse sweep (anodic) cyclic voltammograms are proposed to be due to residual amounts of dioxygen in solution and/or in the protein molecule. Those differences were not seen in the anodic waveforms of simultaneously acquired derivative cyclic voltabsorptometry (DCVA) experiments. Cyanomyoglobin exhibited faster heterogeneous electron transfer kinetics, with a k(o＇) value of 6.0(+/- 1.0)x 10(-4) cm s(-1). Values for the homogeneous rate constants for dissociation (k(f)) and association (k(b)) of cyanide from and to the electrochemically reduced protein were determined to be 0.08(+/- 0.03) s(-1) and 0.06(+/- 0.03) M-1 s(-1), respectively. Imidazolemyoglobin was found to transfer electrons faster than cyanomyoglobin, with a k(o＇) value of 2.0(+/- 0.5) x 10(-3) cm s(-1). The values for the homogeneous rate constants k(f), and k(b) were 0.40(+/- 0.15) s(-1) and 1.1(+/- 0.1) M-1 s(-1), respectively.