text: Photoreduction of CO to formic acid was demonstrated on an p-GaP photocathode in aqueous media. Apart from several other reports of CO photoreduction on p-GaP, there are other p-type semiconductors like p-GaAs, p-InP, p-CdTe, and p/p-Si have been successfully used for photoreduction of CO. The lowest potential for CO photoreduction was observed on p-GaP. This may be due to high photovoltage excepted from higher band gap p-GaP (2.2 eV) photocathode. Apart from formic acid, other products observed for CO photoreduction are formaldehyde, methanol and carbon monoxide. On p-GaP, p-GaAs and p/p-Si photocathode, the main product is formic acid with small amount of formaldehyde and methanol. However, for p-InP and p-CdTe photocathode, both carbon monoxide and formic acid are observed in similar quantities. Mechanism proposed by Hori based on CO reduction on metal electrodes predicts formation of both formic acid (in case of no adsorption of singly reduced CO radical anion to the surface) and carbon monoxide (in case of adsorption of singly reduced CO radical anion to the surface) in aqueous media. This same mechanism can be evoked to explain the formation of mainly formic acid on p-GaP, p-GaAs and p/p-Si photocathode owing to no adsorption of singly reduced CO radical anion to the surface. In case of p-InP and p-CdTe photocathode, partial adsorption of CO radical anion leads to formation of both carbon monoxide and formic acid. Low catalytic current density for CO photoreduction and competitive hydrogen generation are two major drawbacks of this system.