Reaction mechanism of skeletal isomerization of n-butane over Cs2.5H0.5P W12O40 catalyst was studied using 13C MAS NMR.The reaction proceeds mainly via a monomolecular mechanism at 80℃. The contribution of the bimolecular mechanism becomes more significant as the reaction temperature is increased to 150℃. Hydrogen suppresses the bimolecular mechanism, in particular on Pt-promoted Cs2. 5H0. 5P W12 O40 catalyst. The latter catalyst is highly selective to isobutane in the presence of H2 even at 220℃. The kinetics of the monomolecular isomerization reaction and the reaction schemes of 13C scrambling and n-butane isomerization are discussed.