Isotopic analysis of n-butane isomerization over sulfated zirconium oxide, using double C-13-labelled n-butane, shows that at low temperature this isomerization is an inter-molecular process. Probably, a C-8 surface intermediate is formed which isomerizes and undergoes beta-fission; the iso-C-4 fragments are desorbed as i-butane. Previously, the same mechanism was indicated for Fe, Mn promoted catalysts. The isomerization rate at 130 degrees C is drastically lowered by gaseous H-2, because the concentration of the unsaturated species, required for the formation of the C-8 intermediate, is low under such conditions. Whereas C-13-scrambled i-butane is a true primary product, isotopic scrambling of n-butane continues after chemical equilibrium between n-butane and i-butane has almost been reached; i.e. C-13 scrambled n-butane is a secondary product. Intra-molecular rearrangement of carbon atoms in n-butane precedes intermolecular scrambling. The similarity of the isomerization mechanism over unpromoted sulfated zirconia and Fe, Mn promoted sulfated zirconia is paralleled by an equal strength of the acid sites in both catalysts. The shift in the FTIR band of CO adsorbed on the Lewis sites indicates that these sites, presumably surface Zr4+ ions, are weaker acids than Al3+ in dehydrated alumina.