NiO-TiO2/WO3 catalysts was prepared by drying powdered Ni(OH)(2)-Ti(OH)(4) with ammonium metatungstate aqueous solution, followed by calcining in air at high temperature. The presence of NiO and WO3 prevented the phase transition of TiO2 from anatase to rutile forms. From 600 degrees C of calcination temperature, nickel titanium oxide (NiTiO3) was observed due to the reaction between TiO2 and NiO; its amount increased with increasing calcination temperature. Upon the addition of tungsten oxide to titania up to 15 wt.%, the specific surface area and acidity of catalysts increased in proportion to the tungsten oxide content, due to the interaction between tungsten oxide and titania. Since the TiO2 stabilizes the tungsten oxide species, for the samples equal to or less than 25 wt.%, tungsten oxide was well dispersed on the surface of titania, but for the samples containing 25 wt.% or above 25 wt.%, the triclinic phase of WO3 was observed at calcination temperature above 400 degrees C. The high acid strength and acidity was responsible for the W=O bond nature of the complex formed by the interaction between WO3 and TiO2. The catalytic activities for ethylene dimerization were correlated with the acidity of the catalysts measured by the ammonia chemisorption method. Catalysts without NiO or WO3 were inactive as catalysts for ethylene dimerization, because the active sites of ethylene dimerization consist of low valent nickel ion and acidity. (c) 2005 Elsevier B.V. All rights reserved.