The mechanism of n-butane isomerization on a widely studied unpromoted sulfated zirconia catalyst was studied at low conversions in a once-through reactor using 13 C-labeled n-butane as a reactant. The comparison of results from GC/MS analysis of isobutane produced from 12 C containing n-butane and 13 C-labelled n-butane suggests that, at reaction temperatures ≤250°C, the reaction proceeds mainly via the bimolecular mechanism. The decrease in n-butane isomerization activity with time-on-stream did not appear to cause a change in the reaction mechanism.