Microwave-enhance catalytic degradation (MECD) of 4-chlorophenol (4-CP) using nickel oxide had been studied. A nickel hydroxide, Ni(OH)2, was prepared by microwave-assisted heating technique from nickel nitrate aqueous solution and sodium hydroxide (assigned as PM). Then, the as-prepared PM was oxidized by liquid oxidation with sodium hypochlorite (assigned as PMO). Further, pure nanosized nickel oxide was obtained from the PMO by calcination at 300, 400 and 500 degrees C (labeled as C300, C400 and C500, respectively). They were characterized by X-ray (XRD), infrared spectroscopy (IR) and temperature programmed reduction (TPR). Their catalytic activities towards the degradation of 4-CP on the efficiency of the degradation were further investigated under continuous bubbling of air through the liquid phase and quantitative evaluated by high pressure liquid chromatography (HPLC). Also, the effects of initial concentration, pH, temperature and kinds of catalysts on the efficiency of the degradation have been investigated. The relative activity affected significantly with the oxidation state of nickel and decreased slightly with the particle size of nickel oxide, i.e., PMO (> +2) >> C300 (+2, d = 4.1 nm) > C400 (+2, d = 9.4 nm) > C500 (+2, d = 12.0 nm). (C) 2007 Elsevier B.V. All rights reserved.