A series of H-ZSM-5 zeolites with Si/Al ratios between 17 and 170 were tested in the skeletal isomerization of n-butenes to isobutene in the temperature range of 573-773 K. A selectivity to isobutene of ca. 20% was obtained with all zeolites at 573 K independent of the conversion of n-butenes. With increasing temperature the conversion of n-butenes increased on the zeolites with Si/Al ratios up to 70 thigh number of acid sites), whereas the yield of isobutene and the selectivity to isobutene decreased. On the zeolites with higher Si/Al ratios (160 and 170) an opposite behaviour was observed; the conversion of n-butenes decreased while the yield of isobutene and the selectivity to isobutene increased with increasing temperature. A selectivity to isobutene of nearly 90% could be obtained with the latter zeolites at 773 K. These observations are interpreted in such a way that on H-ZSM-5 zeolites with a high number of acid sites, isobutene is formed via the bimolecular mechanism independent of the reaction temperature. On H-ZSM-5 zeolites with a low number of acid sites, a change from the bimolecular to the monomolecular mechanism takes place with increasing temperature.