Two series of catalysts have been prepared by calcination of 12-molybdophosphoric acid or its ammonium salt at various temperatures, in the region of 300-500 degreesC. The catalysts have been characterized by nitrogen adsorption, XRD, FTIR spectroscopy and TG/DTA analysis to delineate changes in the solid phase associated with high-temperature treatment. Ammoxidation of 2-methylpyrazine has been carried out on these catalysts in a continuous, vapor phase, fixed-bed reactor, in the reaction temperature range of 360-420 degreesC. The used catalysts have also been subjected to characterization to ascertain the extent of formation of the salt from its decomposition products. The ammonium salt has shown higher activity, selectivity and better stability, when compared with its parent acid, up to a calcination temperature of 450 degreesC. However, at temperatures higher than the decomposition temperature the activity of the salt decreases considerably, possibly due to the formation of an undesired species mediated by Mo in the reduced state, thus setting a limit to the calcination temperature.