Nickel oxide-silica catalysts were prepared by precipitation from an acidic solution of a nickel salt-sodium silicate mixture. Two types of nickel hydrosilicate, montmorillonite and antigorite are formed in the catalysts. Catalytic activities of nickel silicates for ethylene dimerization and butene isomerization ran parallel when the catalysts are activated by evacuation at elevated temperatures, giving two maxima in activities. The variations in catalytic activities are closely correlated to the acidity of catalysts. The acid site responsible for the catalytic activity is protonic on montmorillonite, while non-protonic on antigorite, as evidenced by the effect of water content and the IR spectra of adsorbed pyridine. Catalytic activities of NiO-TiO2 catalysts modified with H2SO4, H3PO4, H3BO3 and H2SeO4 for ethylene dimerization and butene isomerization were examined. The order of catalytic activities for both reactions was found to be NiO-TiO2/SO42-much greater than NiO-TiO2/PO43- > NiO-TiO2/BO33- > NiO-TiO2/SeO42- > NiO-TiO2, showing clear dependence of catalytic activity upon acid strength. Catalytic activity of nickel sulfate supported on gamma-Al2O3 (NiSO4/gamma-Al2O3) for ethylene dimerization is also closely correlated to the acidity of catalysts, showing that the active sites consist of a low-valent nickel (Ni+) and an acid.