The combined effect of both the water content in the reaction medium and the reaction temperature (in the 400-500 degreesC range) on the acidity deterioration of a HZSM-5 zeolite catalyst in the methanol to olefins process has been studied. Accordingly, the catalyst has been used in reaction-regeneration cycles in an isothermal fixed-bed reactor. The nature of the acid sites has been measured by means of Fourier transform infrared of the pyridine adsorbed. Both the total acidity and the acid strength distribution of the sites have been determined by calorimetric measurements of differential scanning of adsorption of ammonia and temperature-programmed desorption of this base. The results are proof of the irreversible loss of acidity at 450 degreesC for a water content in the feed above 50 wt%. Irreversible deactivation occurs at 500 degreesC from low values of conversion, even when pure methanol is fed.