The effect of different organic solvent + water mixtures on the acid dissociation constants of protonated 5-amino-4-(arylazo)-3-methyl-1-phenylpyrazole derivatives were studied. The organic solvents used are methanol, ethanol, acetone, and dimethylformamide. The results obtained were discussed in terms of the solvent characteristics.The pKBH+ values of the dyes in question were determined and found to depend on both the amount and nature of the organic cosolvent. The decrease of the pKBH+ values as the amount of organic solvent in the medium is increased can be accounted for in terms of the high stabilization of the protonated form of the compounds by dispersion forces rather than by hydrogen bonding. Also the effect of solvent polarizability and hydrogen-bonding interaction on the pKBH+ values and thus on the spectra of the charge-transfer band observed have been discussed. The absolute pKBH+ value for each compound despite the presence of more than one basicity center has also been discussed.