The acid dissociation constants of four hydroxy azo pyrazolopyrimidine derivatives were determined in aqueous-organic solvent mixtures. The organic solvents are methanol, ethanol, acetone, and dimethylformamide. The results obtained are discussed in terms of the solvent characteristics; The ionization constants of the dyes in question depend largely on both the proportion 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. The effect of molecular structure of the azo compound on the pK(a) value is discussed.