The investigation of protonation/deprotonation at carbon is traditionally limited to molecules where acidity has been boosted by introduction of an electron-withdrawing group or by removal of an electron. These restrictions can be removed by application of the laser flash electron photoinjection technique. A thin layer of radicals is initially formed upon reduction of an appropriate substrate by the photoinjected electrons. The time-resolved current-potential responses for the reduction of the radicals thus generated are sensitive to the rate of the protonation of the ensuing carbanion by purposely added acids. The second-order rate constant may then be extracted from the half-wave potential versus time data with satisfactory accuracy in a wide range of values that extends up to the diffusion limit. The method is demonstrated with the example of diphenylmethyl and benzyl carbanions. Several observations may be derived from these first illustrating experiments. There is a large kinetic isotope effect. Proton transfer is intrinsically slow, showing that this property is not the result of the presence of an electron-withdrawing group. The intrinsic barrier is larger in the benzyl case than in the diphenylmethyl case. Unusual temperature effects (negative activation enthalpy) are observed at least in some cases, calling for systematic investigation in future studies.