SO42-/ZrO2 samples were prepared by impregnation of hydrous zirconium oxides with 0.5 M H2SO4 solutions. It was found that the pH of precipitation of the precursor Zr(OH), has a strong influence on the structure of crystalline ZrO2 and on the catalytic properties of the SO42-/ZrO2 for the isomerization of n-butane. The nature and strength of acid sites in SO42-/ZrO2 samples were studied by infrared spectra of absorbed pyridine and by temperature-programmed desorption (TPD) of ammonia. Acidity and catalytic results have been compared with those obtained on an acid zeolite as H-Mordenite. It has been found that infrared spectra of pyridine can not be used for the determination of the superacid character of SO42-/ZrO2 However, the NH3-TPD technique shown that in the case of SO42-/ZrO2 samples there are superacid sites in which the NH3-desorption peak appears at about 815 K. A correlation between the number of those superacid sites and the activity for the isomerization of n-butane has been observed.