A method for the post synthesis modification of molecular sieves with organometallic reagents. The method may be used for large pore molecular sieves and small pore molecular sieves, such as SAPO-34. SAPO-34 is a useful catalyst for the conversion of oxygenates, such as methanol, to olefins. Post synthesis organometallic modification improves catalyst performance and increases light olefin selectivity in the conversion of methanol to olefins.