Benzimidazole and pyridine photodegradation in water was studied in the presence of semiconductor catalysts, such as ZnO, using low-pressure and medium-pressure Hg lamps and sun light for irradiation. Testing of the catalytic efficiency was based on benzimidazole and pyridine removal and decrease in chemical oxygen demand. For benzimidazole, the best results were achieved with ZnO and sun light, for pyridine, with ZnO and low-pressure Hg lamp. Application of methylene blue as a photosensitizer led to a low efficiency in the degradation. Generally, pyridine was less degradable than benzimidazole.