Abstract The effects of different organic solvent-water mixtures on the acidity constants of some azomethines derived from 3-amino-1,2,4-triazol; 2-amino-1,3-pyrimidine and also 2-amino-4-methyl-1,3-pyrimidine have been examined. Two p K a values for the o-OH derivatives were determined; one is assigned to the deprotonation of the nitrogen azomethine group and the other one is ascribed to the ionization of the OH group. The p-OH derivatives give one K a due to the ionization of the OH group. On the other hand, it was observed that with the increase in the amount of organic solvent in the medium, the p K a of the compound studied is decreased. This behaviour can be accounted for in terms of the high stabilization of both the non-protonated and ionic forms of these compounds by dispersion forces rather than by hydrogen bonding. Also, the effects of medium polarizability on the K a values and thus on the spectra of the charge transfer band observed have been discussed.