A mix-valenced nickel oxide was obtained from nickel nitrate aqueous solution through a precipitation with sodium hydroxide and an oxidation by sodium hypochlorite. Furthermore, the oxide was heated under microwave-assisted to fabricate a high-active mix-valenced nickel oxide (assigned as NiOx). Pure nickel oxide was obtained from the NiOx by calcination at 300, 400 and 500 degrees C (labeled as C300, C400 and C500, respectively). They were characterized by thermogravimetry (TG), X-ray diffraction (XRD), infrared spectroscopy (IR), temperature-programmed reduction (TPR), nitrogen adsorption at 196 degrees C and scanning electron microscopy (SEM). Their catalytic activities towards the degradation of 4-chlorophenol were further studied under continuous bubbling of air through the liquid phase. Also, the effects of pH, temperature and kinds of nickel oxide on the efficiency of the microwave-enhanced catalytic degradation (MECD) of 4-chlorophenol have been investigated. The results showed that the 4-chlorophenol had degraded completely into harmless products (CO2 center dot H2O and mineral acids) within 5 min of investigation, under pH 7 and T = 70 degrees C over the fabricated NiOx. (C) 2007 Elsevier B.V. All rights reserved.