The potentiometric titrations of 5-substituted-4-amino-2-methylpyrimidines were examined as a base in various nonaqueous solvents. The quantitative determinations were effectively carried out in acidic and dipolar aprotic solvents. The potentials at half neutralization point (HNP values) were used as a measure of the relative basicity of the pyrimidines in the solvents. The values in each of the solvents could be lineally correlated with the pKa (H₂O) values and the Hammett's σ_m for the 5-substituents. Some of the pyrimidines which deviated considerably from these linear relationships were discussed. Moreover, it was found that the differentiating titration of a mixture of pyrimidines might be facilitated with increasing of the reaction constant of the Hammett's equations. On the other hand, it was indicated by the Hammett's equations and by comparison of ultraviolet absorption spectra that these pyrimidines existed largely as the amino form in the solvents as well as in water and the actual proton acceptor in these molecules was the 1-position of pyrimidine ring.