SO42-/Ti-M-O (M=Al, Fe, Sn) solid superacids were used in the esterification of phthalic anhydride with 2-ethyl hexyl alcohol. The catalytic activity of SO42-/Ti-Al-Sn-O (1) is higher than SO42-/TiO2, and SO42-/Ti-Al-O solid superacids. In the presence of(1) DOP is prepared in good yield at 175 degrees C over a period of 6 h. The acid strength, surface area and catalytic activity of SO42-/Ti-M-O are obviously affected by the preparation conditions. Temperature-programmed desorption of pyridine adsorbed on the catalyst was used to characterize the acid strength distribution of the catalysts. It was found that there were three kinds of acid sites (weak acid, middle acid and superacid) on SO42-/Ti-Al-O, and the concentration of the middle-acid site on the catalysts is directly proportional to its catalytic activity for the esterification. Adding Al into SO42-/TiO2 could increase its surface area greatly.