Medium pore, ZSM-5 zeolite was subjected to chemical vapour deposition to alter its pore opening size. The extent of alteration and its effect on shape selectivity during toluene ethylation has been investigated. The time on stream behaviour of the pore size altered zeolite showed a deactivating trend with enhancement in hours on stream. The stabilization of the alkylation activity of the catalyst has been achieved by controlled coking and the details are discussed.