Nanotube-structured TiO2 electrodes on Ti plates were formed in ethylene glycol solution by the anodic oxidation method applied for different times and calcined at 500 °C. Different amounts of WO3 were decorated on the nanotube surfaces electrochemically. The electrodes were characterized, and the effects of the nanotube length on the Ti plate, decorated WO3 amount, electrolyte concentration, applied potential, and type of radiation source on the oxidation of 3-methylpyridine were investigated, together with the product distribution/selectivity. In a photoelectrocatalytic system, the vitamin B3 yield increased significantly (ca. 17 fold) under UVA by decorating nanotube-structured TiO2 with WO3, whilst low reaction rates and no products were found under Vis irradiation, as only unselective photolytic reactions occurred. This unexpected result was clarified for the first time in the literature.