There are provided a catalyst and a process for converting methanol or dimethyl ether to a product containing C2 to C4 olefins. The catalyst comprises a porous crystalline material having a Diffusion Parameter for 2,2-dimethylbutane of 0.1-20 sec-1 when measured at a temperature of 120 °C and a 2,2-dimethylbutane pressure of 60 torr (8kPa). In addition, the catalyst is characterized by a hydrothermal stability such that, after steaming the catalyst at 1025 °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 °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 °C.