Mechanisms of skeletal isomerization of n-butane over bifunctional catalysts, Pt-Cs2.5H0.5PW12O40 and Pt-sulfated ZrO2, as well as the corresponding solid acids were studied using 1,4-C-13(2)-n-butane. The isotopic distributions of the reactant and product were analyzed with field ionization mass spectrometry, by which the parent peak patterns were obtained. It was found that 1,4-C-13(2)-n-butane was selectively isomerized to C-13(2)-isobutane over these catalysts in the presence of H-2 at 423-523 K, while the corresponding solid acids gave isobutane with binomial distributions of C-13. These results clearly demonstrate that the skeletal isomerization of n-butane proceeded mainly via a monomolecular path with intramolecular rearrangement on both the bifunctional catalysts, while it occurred through a bimolecular path with intermolecular rearrangement on the solid acids. This difference in reaction mechanism is reflected on that in the selectivity to isobutane.