Nitrobenzene "caged" compounds are well-known for their use in delivering biologically active substrates to a reaction mixture after photoexcitation. We have discovered that they also behave as photoreductants from the triplet state after photoexcitation. To explore the properties of these newly discovered photoreductants, a series of substituted nitrobenzene derivatives have been synthesized. These nitrobenzene derivatives exhibit many desirable characteristics suitable for the physiological photoreduction of different proteins, and examples are shown for the photoreduction of cytochrome c and other heme proteins. The observed rate constant for photoreduction of cytochrome c, k(obs,) ranges from 300 to 36 000 s(-1) for the various nitrobenzene derivatives. pH and ionic strength experiments are consistent with a bimolecular reaction wherein the photoreductant and the protein form an electrostatic complex prior to electron transfer. A kinetic model for this bimolecular reaction is described and simulations of the experimental data for the photoreductant 4,5-dimethoxy-2-nitrophenylacetic acid (DMNPAA) yield an inherent unimolecular electron-transfer rate constant (k(et)) of 14 600 s(-1) for the photoreduction of cytochrome c at pH 6.6.