This invention relates to processes for converting oxygenates to olefins that include a step of pretreating molecular sieve used in the conversion reaction with a C.sub.4 C.sub.7 olefin composition, which contains one or more C.sub.4 C.sub.7 olefins. Fresh or regenerated molecular sieve, which is low in carbon content, is contacted or pretreated with the olefin composition to form a hydrocarbon co-catalyst within the pore structure of the molecular sieve, and the pretreated molecular sieve containing the co-catalyst is used to convert oxygenate to a lighter olefin product.