The reaction mechanism of skeletal isomerization of n-butane over typical solid acids such as CS2.5H0.5PW12O40, sulfated ZrO2, and WO3/ZrO2 and their Pt-promoted catalysts have been studied by the use of 1,4-C-13(2)-n-butane. Isotopic distributions in product isobutane and reactant n-butane were quantitatively analyzed by field-ionization mass spectrometry (FI-MS). On these solid acids, below 423 K, isobutane consisted of C-13(0)-C-13(4) isotopes, whose distributions were close to the corresponding binomial distributions, with the reactant n-butane being mainly C-13(2)-n-butane. This indicates that the isomerization proceeded with an intermolecular rearrangement through a bimolecular mechanism. In the temperature range of 493-523 K over these solid acids, the isotopic distributions in isobutane deviated from the binomial distributions; the fractions of C-13(2)-isobutane were greater than those expected from the binomial distributions, showing that an intramolecular (monomolecular) rearrangement became significant. Further, the product isobutane was found to consist of exclusively C-13(2)-isobutane over these Pt-promoted catalysts in the presence of 112, demonstrating the operation of a monomolecular mechanism on the bifunctional catalysts. (C) 2003 Elsevier B.V. All rights reserved.