Kinetic behavior (production of olefins, selectivity) has been studied in the transformation of methanol into olefins on a catalyst prepared based on SAPO-18 in a fluidized bed reactor, for a wide range of operating conditions (temperature, space time, concentration of methanol in the feed, dilution of methanol with water, time on stream). Under certain conditions, an initiation period is observed, which is in agreement with the "carbon pool mechanism" and whose duration largely depends on the operating conditions and, particularly, on water content in the reaction medium. A study has been carried out on the role of water as an inhibiting agent in the formation of olefins and in coke deposition. The methanol/dimethyl ether ratio is higher than that corresponding to thermodynamic equilibrium under conditions in which the catalyst is active for olefin formation. The hydrothermal stability of SAPO-18 has been proven by using the catalyst in a series of 10 successive cycles of reaction-regeneration.