The decomposition of neopentane was studied with and without selective site poisoning with NH 3. The lethal dose, which was just sufficient to kill the catalytic activity, was estimated for each of a series of H-mordenites, HY and HZSM-5 zeolites, all with Si Al ratios higher than 7.0, i.e., over silica-rich compositions where ideally all the sites are supposedly identical. It was found that poisoning no more than 10% of the sites (counted as lattice aluminum ions) was sufficient to eliminate the activity. Some ramifications of these observations are discussed. Unpoisoned and partially poisoned zeolites were investigated using 13C-MASNMR following adsorption of isopropanol-2- 13C. These findings clearly show the efficacy of adsorbed NH 3 in repressing formation of polymeric surface residues.