The catalyst life of zeolites useful in the conversion to olefins of alcohols (e.g. methanol) and/or their ether derivatives (e.g. dimethyl ether) is improved by modifying the zeolite in a 2-stage procedure by providing a controlled low amount of a coke precursor deposit on the external surface of the zeolite, and then heating this treated zeolite in an inert gas at specifically controlled temperatures for a minimum time.