The behavior of 4-amino-5-carboxy-2-methylpyrimidine (ACMP) in glacial acetic acid and the mixed solvent of isopropanol and ethyleneglycol was spectrophotometrically investigated by comparison with those of the ethyl ester and the 1-methyl betaine. The modified Edsall's equation was proposed to calculate potentiometrically the equilibrium constant (K_e value) between the zwitterion and the zwitterion and the uncharged form of ACMP in these solvents. These results indicated that the zwitterion formation of ACMP was more repressed in these solvents than in water. The similar behavior was observed on the cases of 2-aminonicotinic acid (ANA) and 2-aminoisonicotinic acid (AINA) in these solvents. Further, the effect of the dielectric constant of solvent on the zwitterion formation of ACMP, ANA and AINA was spectrophotometrically examined in various alcohols and the mixed solvents of dioxane and water. The zwitterion formation was parallel with increasing of the dielectric constant of solvent.