A series of catalysts, NiSO4/gamma-Al2O3, for ethylene dimerization were prepared by the impregnation method using an aqueous solution of nickel sulfate. Nickel sulfate supported on gamma-Al2O3 was found to be very active even at room temperature, giving a maximum in both activity and acidity when catalyst containing 20% NiSO4 was calcined and evacuated at 600degreesC. The high catalytic activity of NiSO4/gamma-Al2O3 was related to the increase of acidity and acid strength due to the addition of NiSO4. The asymmetric stretching frequency of the S=O bonds for NiSO4/gamma-Al2O3 samples was related to the acidic properties and catalytic activity. That is, the higher the frequency, the larger both the acidity and catalytic activity. The active sites responsible for ethylene dimerization consist of a low-valent nickel, Ni+, and an acid, as evidenced by the IR spectra of CO adsorbed on NiSO4/gamma-Al2O3 and Ni 2p XPS.