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 C-13-labeled n-butane as a reactant. The comparison of results from GC/MS analysis of isobutane produced from C-12 containing n-butane and (13) C-labelled n-butane suggests that, at reaction temperatures less than or equal to 250 degreesC, 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.