The photoelectrocatlytic system is a discharge photocatalytic system aimed at enhancing the photocatalytic performance. In this study, we have obtained experimental data revealing the basic mechanism of the photoelectrocatalytic system. We also report removal of xylene isomers in air using the photoelectrocatalytic system, and these data are compared with our previous data on benzene and toluene. Our analysis based on the Lagmuir-Hinshelwood (L-H) kinetic model shows that the rates of degradation are in the order; benzene < toluene < xylene, in the photoelectrocatalytic system, and explanations are given for the trend.