Mixtures of propionitrile, methanol and water were reacted during different times over a sulfonic acid resin at 15 kg cm(-2) and temperatures from 30 to 93degreesC. Propionitrile hydrolysis was followed by means of FTIR spectroscopy of the catalyst surfaces. In the spectra of used catalysts, different bands appeared that can be attributed to propanamide. The intensity of those bands increased with reaction time and temperature. These observations were consistent with a reaction scheme in which a methyl propionimidate ion is an intermediate species. MTBE synthesis as a test reaction showed that there is a direct relationship between the formation of propanamide and the loss of catalyst activity.