Partial oxidn. of toluene in a fuel cell system with an ionic conductor having catalytic electrodes, partic. an electroconductive carbon substance, esp. oxidised carbon, a metal or a metal cpd., in which a hydrogen donor on an electrode and contacting toluene and oxygen on the other electrode, partic. oxidised carbon whisker, and used for the prodn. of benzaldehyde or cresol.    The reaction carried out in permanganate, nitric acid, bichromate or sulphuric acid aq. soln. is also claimed.    Fuel cell systems prepd. according to the claimed compsns. are used for partial oxidn. of toluene. Metal cpds. used for catalytic electrode includes e.g. halide, nitrate, oxide, hydroxide, phosphate and ammonium complex. The ionic conductor includes e.g. H3PO4, HCl, heteropolyphosphoric acid, H-montmorillonite and Zr phosphate.    USE/ADVANTAGE -   Selective partial oxidn. of toluene to give benzaldehyde or cresol under mild reaction conditions.    In an example, the fuel cell system was prepd. by the following compsns. (a) Oxidn. of carbon material: a carbon material was soaked in 8N-HNO3 and boiled for 2 hrs. washed with water to give an electrode. (b) To prepare the anode: a powder mixt. of 20 mg of Pt, 70 mg of graphite and 5 mg of polytetrafluoroethylene was compressed to give an anode disc. (c) Introduction of hydrogen donor to the anode: An atmospheric mixt. of 50 kPa of H2, 3 kPa of water vapour and Ar for the rest was introduced at a rate of 20 ml/min. (d) Supply of 02: Pure 02 was supplied at a rate of 5 ml/min. in the cathode room contg. 40 ml of toluene.    A disc formed silica wool was immersed with 85% H3PO4 to give an ion conductor. The anode, and a cathode prepd. by compressing a mixt. of 70 mg of oxidised carbon whisker and 5 mg of polytetrafluoroethylene were installed in th respective anode and cathode room. A mixed gas of H2, water vapour and Ar was introduced in the anode room and 02 was introduced in toluene in the cathode room to cause the oxidn. reaction and 22.73 micro mole of benzaldehyde, 2.92 micro mole of benzyl alcohol and 4.69 micro mole of cresol were obtd..