The nature of the active species responsible for butene isomerization over aged LIFER samples is reexamined in the light of the change in the product yields at very short time-on-stream and of the reversible and irreversible increases in weight of the zeolite during the reaction. At very short time-on-stream, the selectivity of butene isomerization is that expected from a dimerization-cracking process, in particular simultaneous formation of isobutene, propene and pentenes. A rapid decrease of all the yields is observed with time-on-stream; however, for isobutene but not for the other products, the initial decrease is followed (after 10 minutes-on-stream) by an increase. The decrease in the yield can be related to the formation of carbonaceous compounds ("coke") which block the access to the pores, while the increase in isobutene yield can be explained by the development of a new isomerization mode which is very selective to isobutene. This new mode could be catalyzed by carbonaceous compounds and/or by reaction products which are shown to be retained inside the pores during the reaction. It is proposed that at short time-on-stream the increase in isobutene yield is due to an autocatalytic reaction, n-butene isomerization occurring on t-butyl carbenium ions formed by adsorption of isobutene molecules (which are slowly desorbed from the pores) on the protonic sites of the zeolite. At long time-on-stream, the active species would be benzylic carbocations formed from carbonaceous compounds trapped in the pores near the outer surface of the crystallites.