Apparent deuterium kinetic isotope effects (KIEapp) of four different methylarene radical cation-pyridine base reactions in dichloromethane (0.2 M tetrabutylammonium hexafluorophosphate) were observed to increase toward a constant value with increasing extent of reaction. The reactions were studied by derivative cyclic voltammetry (DCV), and rate constants were assigned by comparing the experimental with the theoretical DCV data. The kinetic results rule out a simple second-order proton-transfer reaction and implicate a mechanism in which a complex is first formed that then undergoes proton transfer, followed by separation of the products. That KIEapp are extent of reaction-dependent is observed before steady-state is reached. The concurrent analysis of kinetic data for the reactions of both ArCH3.+ and ArCD3.+ with bases under non-steady-state conditions facilitates the resolution of the apparent rate constant [k(app) = k(f)k(p)/(k(b) + k(p))] into the microscopic rate constants (k(f), k(b), and k(p)) for the individual steps. The KIEapp observed during proton-transfer reactions need not be the real kinetic isotope effects (KIEreal). Having access to the microscopic rate constants for the steps in which the proton is transferred allows KIEreal to be evaluated and compared with the corresponding KIEapp. The present study shows that the KIEreal are much greater than the KIEapp derived in the usual way from the rate of the overall reaction.