Over a superacid ZrO2/SO4 system (ZS8 : microcrystalline tetragonal, SO4 content approximate to 2.6 groups per nm(2)), the isomerization of n-butane proceeds rapidly and with high yield of i-butane at 423 K, if the catalyst is calcined in dry air at approximate to 823 K. The active catalyst typically shows : all surface sulfates in a highly covalent form, a high concentration of strong (aprotic) Lewis acid sites, and a fairly low (though never null) concentration of(protonic) Bronsted acid sites. A partial rehydration of the catalyst gradually converts the surface sulfates first into a less covalent form and then into an ionic form, decreases the concentration of strong Lewis sites, and increases (up to a factor of approximate to 10) the concentration of Bronsted sites, while the catalytic activity is gradually extinguished. Initial activity and initial surface features can be recovered by repeating the catalyst activation step.