The effect of time on stream (from 1 min to 44 h) on the rate and selectivity of n-butene isomerization and on the concentration of protonic sites interacting with ammonia or deuteroacetonitrile was determined over a HFER zeolite with a Si/Al ratio of 10. Aging for 44 h was shown to increase the selectivity from 55 to 95% and the turnover frequency for isomerization from similar to 250 to 4000 h(-1). All of this can be explained by considering the HFER pore system as equivalent to a series of noninterconnected nanoreactors (the 10-membered ring channels) into which the reactant molecules have to diffuse without any possibility of desorption before their exit while undergoing various successive bimolecular reactions. In nanoreactors with few protonic sites, n-butene isomerizes selectively through an autocatalytic process; in those with many sites a thermodynamic equilibrium mixture of propene, butenes, pentenes, and deactivating carbonaceous species is formed. (C) 2004 Elsevier Inc. All rights reserved.