Cytochrome c is electroactive at TiO2 modified SnO2 electrodes, displaying quasi-reversible behavior. When water was replaced by glycerol (95%), Edegrees' increased slightly from 254 to 261 mV (NHE). Resonance Raman and visible spectroscopic data indicated that the heme environment was unchanged in glycerol. The thermodynamics of reduction were significantly changed, with DeltaHdegrees'(rc) decreasing from -35.6 to -47.7 kJ mol(-1) and DeltaSdegrees'(rc), from -35.2 to - 76.8 J K-1 mol(-1). The large decrease in DeltaHdegrees'(rc) is a result of exclusion of water molecules from the heme environment, while the decrease in DeltaSdegrees'(rc), can be attributed to the reduced conformational flexibility of the protein in glycerol. The activation energy associated with electron transfer is significantly increased from 5.7 kJ mol(-1) in aqueous buffer to 44.2 kJ mol(-1) in glycerol. These results demonstrate that a relatively polar solvent can significantly affect the redox properties of cytochrome c.