The formation in the channels or cages of zeolites of heavy side products (generally called "coke") is often responsible for their deactivation owing to poisoning of active sites and/or to pore blockage. However, these coke molecules trapped in the zeolite micropores being relatively simple, are not generally inert with respect to the reactants or intermediates of the desired reactions and, hence can significantly affect the activity and selectivity. This participation in catalytic reactions of the coke molecules trapped in the zeolite micropores is shown here in several examples carried out in liquid or in gas phase, with large, medium or small pore molecular sieves: (i) isopropylation of naphthalene and alkylation of toluene with long chain n-alkenes over HFAU and HBEA zeolites; (ii) selective skeletal isomerization of n-butenes over HFER; (iii) selective hydroisomerization of long chain n-alkanes over PtHTON; (iv) selective methanol conversion into light alkenes over SAPO 34. (C) 2002 Elsevier Science B.V. All rights reserved.