The catalytic activity of the zeolite HZSM5 in n-butene conversion was investigated under conditions similar to those of a commercial process for skeletal isomerization. The catalyst was found to be very active in n-butene conversion, but selectivity to isobutene was low due to a predominance of cracking, coking, and oligomerization side reactions. The interaction of 1-butene, cis-2-butene, trans-2-butene, isobutene, sec-butanol, and tert-butanol was also investigated by FT-IR spectroscopy in the temperature range 150-673 K. n-Butenes were first adsorbed on the acidic OHs of the zeolite giving rise to well-defined hydrogen-bonded species. These species probably undergo protonation to the corresponding carbenium ion, which could, however, not be detected, and later, through skeletal isomerization/alkylation/skeletal isomerization sequences, to polyisobutene. In the case of isobutene, a tert-butoxy species supposed to act as the precursor for tert-butyl carbenium ions, active as the cationic polymerization initiator, was detected and characterized spectroscopically. Starting from isobutene, the polymerization is partly hindered at low temperature because of the limited diffusion of the olefin into the zeolite pores. The alcohols dehydrate around room temperature and also give rise to polyisobutene in the same way.