Abstract: The possible effect of catalyst on the kinetics of photoelectroehemieal (PEC) reactions taking place at the surface of semiconductor (SC) electrodes was analysed mainly by considering the nature of junction between SC and catalyst particles. Result of analysis showed that in many cases catalyst could only eatalyse dark reactions. Prerequisite for catalysis of PEC reactions would be sufficiently strong interaction of catalyst particles with the SC/solution junction("liquid junction"), so that the Fermi level of electron system within catalyst particles could approach the quasi-Fermi level of photo-generated minority carriers. Experimental results of investigation of the effects of deposited Au, Ir, Pd and Pt on the kinetics of Ce~(3+)→Ce~(4+) and other reactions taking place on monocrystalline n-TiO2 electrode strongly supported such a model. Technique of RRDE with inter-changeable SC disks was empolyed in investigating catalysis of PEC reactions.