The dissociation constant (pK4) and thermodynamic parameters (pK4) of 8-­aminoquinoline are determined in aqueous and in mixed organic-water (dioxane, EtOH and MeOH) solvent systems. It is observed that ionization constant of 8-aminoquinoline in aqueous system increases with the increase of temperature. The curve is a parabolic one. It is to be noted that pK4 values of this compound are lower than those of quinoline and some of its amino derivatives. For mixed organic-water solvent systems pK4 M and pKaT values versus percent composition decrease slowly with increase in percent of EtOH, MeOH and dioxane. The curve of the pK. versus percent composition is a distorted parabola.The data have been obtained potentiometrically by titrating aqueous 8-aminoquinoline solution with hydrochloric acid. The values of dissociation constant were obtained from these data by a computer program written in OW-BASIC. From pK. values Gibb's free energies (DG) have also been calculated.