The skeletal isomerization of n-butenes on a H-ZSM-5 zeolite with high Si/Al ratio was investigated by using a gradientless recirculation batch reactor. It was concluded that at the very beginning of the reaction, the formation of C-8 surface species should be the first step. These species decompose into propene and pentenes, one part of the just formed and still adsorbed C-3 species immediately dimerizes to hexenes. In the further course of reaction, an oligomerization to C-12 surface species (trimerization) occurs followed by decomposition as shown by the products pentenes, hexenes, and heptenes. The tendency to oligomerization decreased with increasing temperature. For the reaction of lower olefins like propene and ethene, the reaction course was the same as it was observed for n-butene: a gradual increase of the degree of oligomerization with reaction time followed by immediate decomposition. (C) 2003 Elsevier B.V. All rights reserved.