Mechanism of skeletal isomerization of n-butene to isobutene over CoAlPO-11 and H-ferrierite molecular sieves was investigated using C-13 labeled 1-butene. It was confirmed that a high selectivity to isobutene can be reached exclusively via a monomolecular reaction pathway. The bimolecular mechanism is responsible not only for the formation of propylene, pentenes, and other by-products but also for a substantial amount of isobutene. At least 30% of isobutene formed over ferrierite at 620 K was produced via dimerization of butenes followed by isomerization and cracking.