With rising atmospheric CO2 levels, there has been increasing interest in artificial photosynthetic schemes for converting this greenhouse gas into valuable fuels and small organics. Photoelectrochemical schemes for activating the inert CO2 molecule, however, operate at excessive overpotentials and thus do not convert actual light energy to chemical energy. Here we describe the selective conversion of CO2 to methanol at a p-GaP semiconductor electrode with a homogeneous pyridinium ion catalyst, driving the reaction with light energy to yield faradaic efficiencies near 100% at potentials well below the standard potential.