Three Ti/TiO2-RuO2 electrodes were modified with Nb2O5 and used for H acid decompn. in photochem., electrochem. and photoelectrochem. processes in NaClO4 soln. The decompn. course was followed by detn. of COD and total O-2 demand as well as absorbance in UV-VIS spectra. An effect of synergy was obsd. when Ti/TiO2-RuO2-Nb2O5(N) electrode was used in the photoelectrochem. process at 0.3 A and 254 nm.