We describe results of experiment and theory of the cuprous–cupric electron transfer rate in an aqueous solution at a copper electrode. The methods are similar to those we reported earlier for the ferrous–ferric rate. The comparison strongly suggests that, in marked distinction to the ferrous–ferric case, the electron transfer reaction is adiabatic. The model shows that the activation barrier is dominated by the energy required for the ion to approach the electrode, rather than by the energy required for rearrangement of the solvation shell, also in sharp distinction to the case of the ferric–ferrous electron transfer at a gold electrode. Calculated activation barriers based on this image agree with the experimental results reported here.