WO3 decorated TiO2/Ti Nanotube arrays (WO3/TNAs) photoelectrodes were fabricated using in situ anodization and electrochemical deposition method. The morphology and structure of WO3/TNAs photoelectrodes were studied by scanning electron microscopy (SEM) and X-ray diffraction (XRD). The effects of different parameters, such as initial pH, bias potential and anions in solution, on photoelectrocatalytic (PEC) activity of WO3/TNAs photoelectrodes were investigated. The degradation mechanism and degradation products of 4-NP were analyzed using liquid-mass spectrometry (LC/MS). The results showed that the degradation percent of 4-nonylphenol (4-NP) on WO3/TNAs photoelectrodes could reach 90% under neutral conditions after 120 min illumination. After a bias potential of 2.0 V was applied, the degradation percent increased to 98% and the total organic carbon (TOC) removal efficiency was 60%. 4-NP transformed to carboxylic acid substances, CO2 and H2O by hydroxylation, dealkylation role, broken bonds and open-loop. Acid and base as well as SO42-, Cl-, HCO3- and NO3- ions in solution undermined the removal of 4-NP. (C) 2014 Elsevier B.V. All rights reserved.