The catalytic activities of a series of alkali and alkaline earth cation exchanged Y, FAU zeolites were investigated in the non-thermal plasma-assisted NOx reduction reaction using a simulated diesel engine exhaust gas mixture. The catalytic activity of the Y, FAU zeolite showed significant variations with both the nature of the charge compensating cation, and the method of catalyst preparation. Our results show that conventional multiple solution ion exchange is insufficient to prepare the most active catalyst for the given cationic form. The highest NOx conversion level was achieved over a Ba-Y, FAU which was prepared by a multiple ion exchange method, in which each solution ion exchange step was followed by a high temperature calcination. A systematic change in the catalytic activity was observed as a function of the charge density around the charge compensating cation. For both catalyst series (alkali and alkaline earth ion exchanged Y, FAU), the specific activity decreased with increasing electrostatic field around the charge compensating cation. The large difference in the NOx reduction activity at a given e/r ratio. however, may suggest different reaction mechanisms for the two sets of catalysts. Indeed, there is a noticeable difference in the product distribution (selectivity) for the alkali and alkaline earth series of catalysts. Our results also reveal that extreme care must be taken when catalytic activities are compared for seemingly similar materials. We found that two base zeolite materials with identical Si/Al ratios, obtained from the same manufacturer but from different synthesis batches show significantly different catalytic behavior. (C) 2003 Published by Elsevier B.V.