The proton form of a natural zeolite, clinoptilolite, was found to be an efficient catalyst for the skeletal isomerization of n-butenes to isobutene. The acidity and characteristic pore structure of this zeolite appeared to be responsible for the good performance. The effects of process variables were studied. When reaction conditions were chosen such that the dimerization reaction, the major side reaction, was suppressed, high selectivities to isobutene were obtained.