Photoelectrochemical CO2 reduction was examined in a high-pressure (40 atm) CO2 + methanol medium using the p-type semiconductor electrodes p-InP, p-GaAs, and p-Si. With the p-InP photocathodes, current densities up to 200 mA cm(-2) were achieved, with current efficiencies of over 90% for CO production, while hydrogen gas evolution was suppressed to low levels. At high current densities and CO2 pressures, the CO2 reduction current was found to be limited principally by light intensity. Of the various factors that were found to influence the product distribution, including the concentrations of added water and strong acid, CO2 pressure was the most critical factor. We propose that the adsorbed (CO2)(2)(.-) radical anion complex reaches high coverages at high CO2 pressures and is responsible for both the high current efficiencies observed for CO production and the low values observed for H-2 evolution. Furthermore, we propose that this adsorbed complex is responsible for stabilizing all three semiconductor electrode materials at high CO2 pressures, even at current densities as high as 100 mA cm(-2).