Reduced CO2 was found to be formed from CO2 on Pd, and the characteristics of this reaction were studied in comparison with the reaction on Pt in phosphate buffer solution (pH 6.4) at room temperature by anodic stripping voltammetry (ASV), in situ Fourier transform IR reflection absorption spectroscopy (in situ FTIRAS), and Tafel plots. The most positive potentials for the formation of reduced CO2 on Pd and Pt were found to be 0.05 V and 0.3 V (vs. RHE), respectively. Reduced CO2 was assigned as adsorbed CO on both electrodes by in situ FTIRAS. The adsorption modes were bridged and three-fold CO on Pd, whereas linear CO was found on Pt. The adsorption affinity of reduced CO2 at negative potentials on Pt was higher than that on Pd. This suggests that reduced CO2 behaves as a catalytic self-poison for the electrochemical reduction of CO2 on Pt, but it partly becomes an intermediate in the reduction of CO2 on Pd.