Picosecond absorption spectroscopy has been employed in the study of the dynamics of proton transfer within substituted benzophenones/N,N-dimethylaniline contact radical ion pairs. The reactions were investigated in the solvents cyclohexane, benzene, and dimethylformamide. The correlation of the reaction rates with the change in free energy reveals that the reaction pathway corresponds to a nonadiabatic process, that is the reaction proceeds by proton tunneling. In nonpolar solvents, an "inverted region" is observed in the proton-transfer process.