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. 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.