The formation of carbonaceous compounds ("coke") from propene and isobutene on a 5A zeolite and their effects on the capacity for nitrogen adsorption were investigated in a microbalance system. The operating conditions of coking were as follows : 100-420 degrees C, pressure of alkene of 0.97 bar, 0.085 g of the 5A zeolite, and operating time from 5 min to 100 h. At low temperatures (100-150 degrees C), coke constituted mainly of oligomers formed on the acid sites of the zeolite. These oligomers located in the ct cages can be removed by thermal treatment in vacuum at high temperatures. The effect on the adsorption capacity was more pronounced for coke formed from isobutene due to the preferential location of oligomers in a cages near the outer surface of the crystallites. At high temperatures (350-420 degrees C), aromatics and polyaromatics were formed which cannot be removed by a simple thermal treatment. The effect of coke on the adsorption capacity showed that whatever their origin, these coke molecules (most likely the polyaromatic ones) blocked the access of adsorbates to the zeolite pores.