In this research work the deactivation of an H-MFI (Si/Al-fr = 12) zeolite was studied during the methylcyclohexane transformation at 350 degrees C. Besides the normal activity decay with time on stream due to the formation of coke molecules, the poisoning with three nitrogen basic compounds (3-methylpyridine, quinoline and 2,6-dimethylpyridine) was studied. All three bases cause a strong decrease in conversion. The decrease in activity is proportional to the amount of nitrogen retained in the zeolite. On the other hand, the intrinsic poisoning effect of the bases seems to correlate well with its proton affinity, following the order: 2,6-dimethylpyridine > quinoline > 3-methylpyridine. Although the relatively small pores of the MFI structure. until a saturation amount, the total injected amount of nitrogen bases stays in the zeolite in the protonated form. Even so, results show that the bulkier base cannot penetrate as deep into zeolites crystallites and accumulates on the external surface which can partially explain its higher poisoning ability. Besides the actual bases, no nitrogen coke molecules were found by GC-MS coupling; soluble coke molecules were polyaromatics with 2-4 rings. (c) 2006 Elsevier B.V. All rights reserved.