We have carried out a theoretical study, using semi-empirical methods, on the tautomerism of 2-methylthio-6-amino-4oxo-pyrimidine (S6AMP) and 2-methoxy-6-amino-4-oxo-pyrimidine (O6AMP), both in the gas phase and aqueous solution. The results shows that in the gas phase and solution the oxo-amino tautomer is the most stable specie, in both compounds. These compounds and N-methyl derivatives were caracterizaded by N-15-NMR. The spectrums of S6AMP and O6AMP in trifluoracetic acid indicate that the N1 of pyrimidinic ring is the preferential site of protonation. This result is in keeping with the values of protonic affinities, calculated by AM1 method.