Transformation of linear butenes and iso-butene was carried out over H-FER (Si/Al ratio of 8) in a wide temperature and pressure range and with varying contact times. Catalyst lifetime over 100 h could be achieved by eliminating small traces of contamination, which accelerate deactivation, from the feed. The kinetic analysis revealed a close connection between iso-butene and octene formation and suggested a different pathway for propene and pentenes than for iso-butene. Low-pressure experiments showed very high initial selectivity to iso-butene at 40 mol% conversion, suggesting that coke does not play a significant role in the selective skeletal isomerization leading to iso-butene. Strong desorption limitations seem to exist, which are gradually reduced at higher temperatures. A new approach of the operational mechanism is introduced, where the regain of intrinsic selectivity of FER is proposed to occur by deactivation of the coke deposited in the pores. (C) 2001 Academic Press.