Fast hydrogen atom transfers from various methylbenzenes (ArH) to photoactivated quinones Q* show primary kinetic isotope effects k(H)/k(D) of 2.4-5.6. The quantitative effects of added inert salt on the kinetics and on the yields of the intermediate cation radical ArH+. demonstrate that hydrogen transfer proceeds via a two-step sequence involving an initial electron transfer to form the ion-radical pair [ArH+., Q(-.)] which subsequently undergoes proton transfer according to the electron-transfer mechanism in Scheme 1.