Abstract: n-TiO₂ electrodes covered with traces of Pt and Ag were studied by measuring polarization curves both under illumination and in the dark in 1N NaOH solution flushed with O₂, or Ar.Experimental results showed that, in solution saturated with O₂, reduction of oxygen takes place on noble metal.The rate of the reaction is not affected by illumination of the electrode.In this case, photooxidation of water or other components takes place on the part of electrode surface which is not covered with noble metal.In other words, these two reactions act as a pair of conjugated reactions, just as a photoelectrochemical cell under short circuit.The rate of conjugated reactions is usually controlled by the rate of reduction of oxygen, which is favoured by the increase of the amount of noble methal on the surface of the semiconductor electrode.The current that flows through outer circuit is the algebraical sum of the partial currents of these two reactions.Eesults of interfacial impedance measurement and the analysis of polarization curves suggested that the type of contact between Pt or Ag and n-TiO₂ substrate is mainly ohmic.Therefore, the position of Fermi level of electron system in noble metal is the same as that in the bulk of semiconductor electrode.Photooxidation of CN^-and (C₂H₅)₂NH was carried out in solution containing TiO₂ powder covered with noble metals.The rate of photooxidation could be explained by the above suggested mechanism.