The influence of organic solvents on the electrooxidation of cinnamyl alcohol on Mn,Pt-Nafion has been investigated. Though tetrahydrofuran (THF) gave a high current efficiency (90%), carbon tetrachloride gave a low current efficiency (< 20%). The importance of a co-solvent for the SPE electrolyzer was proposed and substrate transport to the active site located at the interface platinum/ionic cluster region was enhanced by solvents miscible with water. Though THF behaved as a good co-solvent, carbon tetrachloride did not. A composite electrode deposited with platinum on both surfaces of Nafion was applied to oxidation. The cell voltage was as low as 2 V.