The dynamics of proton transfer within a variety of substituted benzophenones/N,N-dimethylaniline contact radical ion pairs are examined in a wide range of solvent polarities. The correlation of the rate constants with the thermodynamic driving force reveals both a normal and inverted region for proton transfer in solvents with an E(T)30 value of less than 43.1; in solvents with E(T)30 greater than 43.8, only the normal region is observed. Also, the kinetic deuterium isotope effect is examined. The solvent and isotope dependence for the transfer process is examined within the context of the Lee-Hynes model for nonadiabatic proton transfer. The theoretical analysis of the experimental data suggests that the reaction path for proton transfer involves tunneling. Conventional transition state theory with the inclusion of tunneling in the region of the transition state cannot account for the observed kinetic behavior.