Several sulfate-doped ZrO2 catalysts (s.d.-ZrO2) have been prepared by sulfating, with either sulfuric acid or ammonium sulfate, samples of monoclinic ZrO2, Y2O3-stabilized tetragonal ZrO2, and amorphous Zr hydroxide, and by calcining the sulfated precursors at T greater than or equal to 673 K. We then examined (by TEM, XRD, FTIR, and catalytic tests) the effect of sulfation on ZrO2 crystal phase and morphology, the effect of ZrO2 crystal phase on the spectroscopic features of the surface sulfate groups, and the catalytic behaviour of the three families of s.d.-ZrO2 systems in the isomerization of n-butane. It could be concluded that the sulfated monoclinic ZrO2 phase is catalytically not very active (if at all), the nonstabilized tetragonal ZrO2 phase is unstable to the sulfation process, and the sulfated tetragonal ZrO2 phase is catalytically active, no matter what the preparative route has been, provided that the sulfated system is calcined at T greater than or equal to 823 K. The latter condition is discussed in terms of the structural and spectroscopic features of the surface sulfate layer.