Using C-13 MAS NMR, conversion of selectively C-13-labeled n-butane on sulfated zirconia catalyst has been demonstrated to proceed initially via two parallel routes: scrambling of the selective 13C label in the n-butane molecule and selective formation of isobutane. The combination of the results obtained by both in situ 13C MAS NMR and ex situ GC-MS analysis provides evidence for the monomolecular mechanism of the C-13-label scrambling, whereas isomerization into isobutane proceeds through a pure bimolecular mechanism. Further, the intermolecular mechanism of n-butane isomerization is complicated and turns into conjunct polymerization. Besides isobutane, conjunct polymerization gives also the products of butane disproportionation, propane and pentanes, as well as the stable cyclopentenyl cations; the latter may be in charge of catalyst deactivation. (C) 2003 Elsevier Inc. All rights reserved.