The deactivation of rare-earth-exchanged zeolite X as catalysts in isobutane/2-butene alkylation between 40 and 130 degreesC was studied. The deactivated samples and the isolated deposits were analyzed by a range of techniques, including for the first time (matrix assisted) laser desorption/ionization time-of-flight mass spectroscopy (MALDI-TOF MS, LDI-TOF MS). The compounds found in the deactivated zeolites are, in addition to large alkanes and alkenes, highly unsaturated and highly branched species containing cyclic structures, which are increasingly aromatic as the reaction temperature increased. The deposits in part interact strongly with the acid sites and block the sites for further alkylation reactions. Their structure and route of formation resemble those of conjunct polymers formed in liquid acid-catalyzed alkylation. (C) 2003 Elsevier Inc. All rights reserved.