Isotopic labeling with deuterium of the surface of a sulfated zirconia catalyst that was precalcined at 650 degrees C and evacuated at 450 degrees C, shows that this surface contains a small number of Bronsted sites; the number ratio of sulfate ions to Bronsted sites is 12.5/1. Isomerization of n-butane to iso-butane over this catalyst with a C-4 feed from which olefins and dienes were rigorously removed (estimated olefin/paraffin ratio, 10(-15)) is slow and so is the rate of catalyst deactivation. After 25 min reaction the turnover frequency, based on the number of surface protons detected by exchange against deuterium, is similar to 10(-4) s(-1). At a conversion of 6.6% n-butane to iso-butane, each surface site has turned over one molecule; all D atoms have been exchanged against H atoms from the butane. The iso-C-4 product initially contains roughly 20% iso-C-4-d(1). At the same stage, the n-C-4 contains 4% n-C-4-d(1). In view of the large n-C-4/iso-C-4 ratio at this stage, this means that more D atoms have moved into n-C-4 molecules than into iso-C-4 molecules. This indicates the participation of surface Bronsted sites in exchange reactions and events such as the previously detected (V. Adeeva et al. Catal. Lett. 33, 135 (1995)) intramolecular rearrangement of C atoms in n-butane via a protonated cyclopropane intermediate. The absence of any molecules with more than one D atom indicates that all olefin molecules at the surface of this catalyst are tied up as "carbenium ions" or as surface alkoxy groups.