Mechanism of skeletal isomerization of n-butane over Cs2.5H0.5PW12O40 as well as sulfated ZrO2 was revealed at a temperature range 393 - 523 K by using 1,4-C-13(2)-n-butane. C-13-Distributions of isobutane at 393 K were close to binomial distribution, indicating a bimolecular pathway. On the other hand, an intramolecular (monomolecular) rearrangement became significant over Cs2.5H0.5PW12O40 at 523 K.