The transformation of methanol into olefins over SAPO-18 catalyst in the 250-450 degrees C range has been studied. The process has been analyzed by following the evolution with time on stream of the mass deposited on the catalyst, of the heat flow evolved, and of the product formation, which are analyzed by means of on-line mass spectroscopy. The results are in agreement with the "hydrocarbon pool" mechanism, and show that the process is conditioned by an initiation step in which the reactive intermediates for the production of olefins are generated. These intermediates are trapped in the SAPO-18 cages. As time on stream is increased, the steps of initiation, maximum production of olefins, and deactivation develop, and the duration of these steps depends on reaction conditions. Water content in the feed has an important attenuating effect on the methanol adsorption and delays the generation of the active intermediates, and also attenuates the subsequent degradation of the intermediates to coke. This attenuation of coke formation is of great importance for reducing catalyst deactivation.