The effect of five water-miscible organic solvents (tetrahydrofuran, N,N-dimethylformamide, acetonitrile, 2-propanol, and methanol) on the oxidation of pinacyanol chloride (Quinaldine Blue) by horse heart cytochrome c was determined. Hydrogen peroxide was used as the oxidant, and a change in catalytic property of the dissolved protein was observed after a certain threshold concentration of the organic solvent had been reached. The maximum specific activity was correlated with the Dimroth-Reichardt parameter for the solvents, which is directly related to the free energy of the solvation process. The kinetic constants for the oxidation of pinacyanol chloride were determined in systems containing different proportions of tetrahydrofuran. The best catalytic efficiency (kcat/KM,app) was obtained in a system containing 50% tetrahydrofuran in phosphate buffer. In a mixture containing 90% tetrahydrofuran, cytochrome c showed 18% of its maximum activity. The inactivation of cytochrome c was mainly due to the presence of hydrogen peroxide, and a direct correlation was found between the inactivation constant and the concentration of hydrogen peroxide in the system. The chemical modifications and immobilization of cytochrome c were able to change its biocatalytic activity and stability in the organic solvent system. The kinetic constants and the inactivation of three other type c cytochromes, from Saccharomyces cerevisiae, Pseudomonas aeruginosa, and Desulfovibrio vulgaris Hildenborough in a system containing 90% tetrahydrofuran were compared with those of cytochrome c from horse heart. Cytochrome c551 from P. aeruginosa showed the best stability against hydrogen peroxide and a higher catalytic efficiency than that of horse heart cytochrome c.