The acid dissociation constants of 5-(substituted phenylazo) barbituric and thiobarbituric acids were determined in aqueous media and in 25–75% (v/v) ethanol–water and dioxane–water mixtures potentiometrically in the temperature range 25–40 °C, in presence of 0.5 M KCl. The values of the enthalpy change ΔH (kJ mol