Nanophases of sol-gel zirconia were prepared with HCl, C2H4O2 and NH4OH as hydrolysis catalysts, and sulfated with H2SO4. They were analyzed by using X-ray powder diffraction, and their crystalline structure was refined by using the Rietveld method. All samples annealed below 300 °C were amorphous. The non-sulfated samples crystallized around 350 °C, while the sulfated samples crystallized around 600 °C, when they started loosing sulfate ions. In the initial stage of crystallization, both the tetragonal and monoclinic nanophases coexisted, with the tetragonal as the main phase. Annealing the samples at higher temperatures transformed the tetragonal nanophase, stabilized by OH ions, into the monoclinic one.