Pd/C catalyst was prepared by hydrogen reduction method and used for the Pd/C gas-diffusion electrode. It was characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS) and cyclic voltammetry (CV) techniques. The electrochemical degradation of 4-chlorophenol was investigated in a diaphragm electrolysis device, by two different feeding gas modes, using the Pd/C gas-diffusion electrode and the carbon/polytetrafluoroethylene (C/PTFE) gas-diffusion electrode as a cathode, respectively. The results indicated that Pd particles with an average size of 4.0 nm were highly dispersed in the activated carbon with an amorphous structure; Pd content on the surface of the Pd/C catalyst reached 1.3 at.%. Furthermore, feeding with hydrogen gas firstly and then with air was in favor of improving 4-chlorophenol removal efficiency. The Pd/C gas-diffusion cathode can not only reductively dechlorinate 4-chlorophenols by feeding hydrogen gas, but also accelerate the two-electron reduction Of 02 to hydrogen peroxide (H2O2) by feeding air. Therefore, the removal efficiency of 4-chlorophenol by using the Pd/C gas-diffusion cathode was better than that of the C/PTFE gas-diffusion cathode. And both the removal efficiency and the dechlorination degree of 4-chlorophenol reached about 100% after 60 min, and the average removal efficiency of 4-chlorophenol in terms of chemical oxygen demand (COD) exceeded 70% after 120 min. The analysis of high-performance liquid chromatography (HPLC) identified that phenol was the dechlorination product, and hydroquinone, benzoquinone, maleic, fumaric, crylic, malonic, oxalic, acetic and formic acids were the main oxidation intermediates. A reaction pathway involving all these intermediates was proposed. (C) 2007 Elsevier B.V. All rights reserved.