Product selectivity for the electrochemical reduction of aromatic ketones was found to be controlled on a semiconducting p-Si and p-GaAs electrodes by illumination. The selectivity control was neither induced on semiconducting n-Si and conducting metal (Sn) electrodes under illumination nor in the reduction of an aliphatic ketone on the p-Si electrode. It was concluded that the excitation of both the semiconducting electrodes and ketone molecules are inevitably required for the selectivity control.