A procedure has been developed which allows growth of the zeolite ZSM-5 in chrysotile (asbestos) fibers which have previously had magnesium partially leached out. The resulting materials, when used as catalysts in the conversion of methanol into hydrocarbons showed very interesting product selectivities: i.e., (a) at high magnesium leaching degree (MLD), relatively larger amounts of aromatics and isoparaffins were produced; and (b) at low MLD, light olefins were the main products in the gaseous phaseFrom NH3 absorption/temperature programmed desorption studies, it was suggested that at lower MLD (higher magnesium content) the dual acidic and basic adsorption sites were responsible for the enhanced production of light olefins from methanol conversion