A series of catalysts, NiSO4/SiO2, for ethylene dimerization was prepared by the impregnation method using an aqueous solution of nickel sulfate. No diffraction line of nickel sulfate was observed up to 20 wt%, indicating good dispersion of nickel sulfate on the surface of SiO2 and SiO2-Al2O3. The addition of Al2O3 to NiSO4/SiO2 improved the thermal stability of catalyst and increased the number of acid sites on the surface of catalyst. Consequently, the Al2O3-promoted catalyst exhibited remarkably higher catalytic activity than non-promoted NiSO4/SiO2. The catalytic activities of supported NiSO4 catalysts were correlated with the acidity of catalysts measured by the ammonia chemisorption method, regardless of the kind of support (SiO2 or SiO2-Al2O3). (c) 2006 Elsevier B.V. All rights reserved.