There is provided a catalyst and a process for converting methanol or dimethyl ether to a product containing C.sub.2 to C.sub.4 olefins. The catalyst comprises a porous crystalline material having a Diffusion Parameter for 2,2-dimethylbutane of about 0.1-20 sec.sup.-1 when measured at a temperature of 120.degree. C. and a 2,2-dimethylbutane pressure of 60 torr (8 kPa). In addition, the catalyst is characterized by a hydrothermal stability such that, after steaming the catalyst at 1025.degree. C. for 45 minutes in 1 atmosphere steam, the catalyst exhibits a methanol conversion activity of at least 50% when contacted with methanol at a methanol partial pressure of 1 atmosphere, a temperature of 430.degree. C. and 0.5 WHSV. The porous crystalline material is preferably a medium-pore zeolite, particularly ZSM-5, which contains phosphorus and has been severely steamed at a temperature of at least 950.degree. C.