Several CaO-ZrO2 samples have been prepared by impregnation of sulfated zirconia with calcium nitrate solutions followed by calcination at 1000 K for 3 h. The addition of calcium oxide to the sulfated zirconia contributes to the stabilization of the tetragonal form of zirconium oxide, although the major phase present, in the CaO-ZrO2 samples is the monoclinic one. Calcium oxide causes the transformation of the surface sulfate groups of sulfated zirconia from covalent to ionic and the shift of the O-H stretching bands to higher wavenumbers, reaching values higher than those corresponding to pure ZrO2. in this way, calcium oxide modifies the acid character of sulfated zirconia; the strongest Lewis acid sites were observed to disappear progressively with increasing calcium content. Bronsted acidity was also found to disappear when the molar concentration of calcium oxide reaches 3%.