The reaction of acetonitrile, methanol and water over a commercial ionic exchange sulfonic acid resin was conducted in a batch reactor for different reaction times at 1.47 x 10(5) Pa and temperatures from 303 to 366 K in order to study the hydrolysis of acetonitrile and its poisoning effect. The FTIR spectra of the variously used and the fresh catalysts were compared to observe that bands appeared on the spent catalysts that can be unambiguously attributed to acetamide on the surface. The intensity of the bands increased as a function of reaction time at a given temperature, and as a function of temperature at a given reaction time. A reaction scheme consistent with these observations was proposed to account for acetamide formation, in which a methyl acetimidate ion is an intermediate species. The formation of poisoning ammonium ions cannot be disregarded. The synthesis of MTBE from methanol and isobutylene over poisoned catalysts showed that there is a direct relationship between acetamide yield and the loss of catalyst activity.