The photoelectrocatalytic oxidation of potassium hydrogen phthalate (KHP) at a titanium dioxide (TiO2) nanoparticle electrode was investigated in the presence of phosphate, which is usually considered an inhibitor of the photocatalytic process. Self-inhibition was observed during the photoelectrocatalytic degradation of KHP in the electrolyte solution with a little and without phosphate. This self-inhibition phenomenon cannot be eliminated by improving the light intensity but can be improved in the presence of a specified amount of phosphate, which extends the detection range of KHP. The adsorption of KHP indicated that the effective separation of KHP molecules adsorbed on the surface of the TiO2 electrode is a key prerequisite for eliminating the self-inhibition phenomenon. It is well-known that phthalic acid is one of the principal intermediates in the photocatalytic degradation of polynuclear aromatic hydrocarbons. Therefore, this study can provide a solution to the self-inhibition that occurs during the photocatalytic/photoelectrocatalytic degradation of polynuclear aromatic hydrocarbons. More importantly, this study can also provide an approach to extending the detection range of chemical oxygen demand using the photoelectrocatalytic process in the presence of polynuclear aromatic hydrocarbons. (C) 2014 Elsevier B.V. All rights reserved.