On a fresh BEA zeolite, skeletal isomerization of 1-pentene is accompanied by various side reactions, such as dimerization-cracking, hydrogen transfer and coking. Coking causes a significant increase in skeletal isomerization and a decrease in the other reactions. IR spectroscopy from samples after 95 min time on stream shows a decrease of Bronsted acid sites of 93 and 28% for the silanol groups. This is in agreement with TPD measurements of ammonia and especially pyridine on fresh and used BEA zeolite. It can be seen that the concentration and strength of acid sites decreased. Acid sites close to the pore mouth of BEA are discussed as active sites for isomerization catalysis. In contrary to BEA, on ferrierite (FER) side reactions are negligible and a high selectivity to iso-pentenes is observed. The restricted pore size of FER and the monomolecular pathway of n-pentene isomerization can explain this fact. Deactivation is more pronounced at lower temperatures for both catalysts. (C) 2003 Elsevier Science B.V. All rights reserved.