Photoelectrochemical degradation of a textile diazo dye, naphthol blue black (NBB), has been investigated using different semiconductor electrodes. A higher photoelectrocatalytic activity has been observed for WO3 film electrodes, prepared by electrodeposition, than for TiO2 nanoparticulate film electrodes. By annealing of the WO3 film deposited on a Pt substrate, we were able to increase the photodegradation rate of NBB, owing to the reduced losses due to the electron-hole recombination. This treatment also increased the long-term photostability of the electrode. The effect of the supporting electrolyte was examined using NaCl, NaClO4, KNO3, and Na2SO4 solutions. The highest NBB degradation rate was found in the acidic Cl- medium. With increasing solution pH, the degradation rate decreased to a lower level indicating a change in the NBB photodecomposition mechanism. The reaction is of the first order with respect to NBB in the concentration range up to ca. 6 x 10(-5) M NBB. The apparent rate constants for the photoelectrocatalytic degradation of NBB in 0.5 M NaCl + 8.8 x 10(-5) M NBB solution, on a WO3 film electrode at E = 1.08 V Versus SCE, are 1.302 x 10(-4) and 3.41 x 10(-5) cm(-2) s(-1), under illumination with ultraviolet and visible light, respectively. It has been found that the mechanism of the sub-band excitation with the dye self-sensitization is a minor degradation path for NBB, while the main path involves the electron-hole generation and heterogeneous oxidation with participation of valence-band electron holes. The latter reactions involve oxidation of solution components with generation of powerful oxidants, such as the OH., Cl-2(.-), and Cl-. radicals, being directly responsible for the fast dye decomposition.