The effects of solvents on the electroreduction of nitrobenzene on Cu, Pt-Nafion were investigated. The presence o a sufficient amount of a cosolvent in the catholyte or in the anolyte facilitated the reduction of nitrobenzene to aniline. Without a sufficient amount of a cosolvent, the mass transfer of nitrobenzene to the active sites on the cathode was suppressed, and hydrogen evolution became significant. The solubility parameter value, delta(mix), of the catholyte is an important index for predicting the effectiveness of mass transfer of nitrobenzene to the active sites of the solid polymer electrolyte composite electrode. When an aqueous solution was used as the anolyte, catholytes with delta(mix) > 11 cal1/2 cm-3/2 were effective for aniline production by the electroreduction of nitrobenzene on Cu, Pt-Nafion.