The acid dissociation constants of some new 8-[(aryl)azo]-2,4-dimethylpyrazolo[1,5-alpha]pyrimidine-7(6H)-one derivatives were determined in aqueous-organic solvent mixtures. The organic solvents are methanol, ethanol, acetone, and dimethylformamide. The results obtained were discussed in terms of the solvent characteristics. The ionization constants of the title dyes depend largely on both the ratio and the nature of the organic cosolvent. Hydrogen-bonding interactions of the conjugate base with solvent molecules as well as the solvent basicity contribute the major effects on the ionization process. It has been concluded that the pK(a) values increase by increasing the amount of organic cosolvent in the medium. The effect of molecular structure of the azo compound on the pK(a) value is discussed.