A series of catalysts, NiSO4/Al2O3-TiO2, for acid catalysis was prepared by the impregnation method, where support, Al2O3-TiO2 was prepared by the coprecipitation method using a mixed aqueous solution of titanium tetrachloride and aluminum nitrate solution followed by adding an aqueous ammonia solution. The addition of nickel sulfate (or Al2O3) to TiO2 shifted the phase transition of TiO2 from amorphous to anatase to higher temperature because of the interaction between nickel sulfate (or Al2O3) and TiO2. 15-NiSO4/5-Al2O3-TiO2 containing 15 wt% NiSO4 and 5 mol% Al2O3, and calcined at 400 degrees C exhibited maximum catalytic activities for both reactions, 2-propanol dehydration and cumene dealkylation. The catalytic activities for both reactions were correlated with the acidity of catalysts measured by the ammonia chemisorption method. The charge transfer from Ti atoms to the neighboring Al atoms strengthens the Al-O bond between Al and the surface sulfate species. The addition of Al2O3 up to 5 mol% enhanced the acidity, thermal property, and catalytic activities of NiSO4/Al2O3-TiO2 gradually due to the interaction between Al2O3 and TiO2 and consequent formation of Al-O-Ti bond.