The role of acid strength in skeletal isomerisation of n-butene to isobutene was studied on a series of open-surface catalysts and several ZSM-5 samples. The highest selectivity was obtained on the open-surface catalysts of moderate acid strength (Ho ca. -5). When the acidity is lower, activity of the catalysts is not sufficient, when the acidity is too high, oligomerisation and by-product formation are induced. In this study, only limited modification of the acidity of zeolites had been achieved and therefore, the effects on butene isomerisation were not very pronounced and, they were observable only at high temperatures (ca. 750 K).