Adsorption features of CO(ads) and reduced CO2 on a polycrystalline platinum electrode in phosphate buffer solution were investigated by anodic stripping voltammetry (ASV) and in-situ Fourier transform IR (FTIR) measurement. The ASV peak of CO(ads) at 0.45 V (with respect to a reversible hydrogen electrode did not appear in the anodic stripping voltammogram of reduced CO2. However, sequential CO2/CO dosing at 0.05 V added the ASV peak at 0.41 V to the anodic stripping voltammogram of reduced CO2. The ASV peaks at 0.45 and 0.41 V were assigned to bridged CO by IR bands around 1850 cm-1, similar to IR bands in acidic solution. The sequential CO2/CO dosing gave an IR band of bridged CO of 1842 cm-1 at 0.05 V. However, the adsorption of reduced CO2 did not give a strong IR band of bridged CO. The potential dependence of the IR band feature of linear CO from CO gas was different from that from reduced CO2. The electron numbers per site for CO(ads) and reduced CO2 oxidation processes were discussed in comparison with the results of ASV and FTIR.