The conditional acidity constants, pK(a), of methyl red and phenol red were determined in (10, 30, and 50) wt % binary aqueous mixtures of methanol and ethanol at 298.15 K and at constant low ionic strength (2.5 . 10(-3) mol . dm(-3)) by a spectrophotometric method. For methyl red and phenol red and in the studied cosolvent + water systems, pK(a) ranged from 3.78 +/- 0.06 to 5.50 +/- 0.04 and 8.50 +/- 0.1 to 6.71 +/- 0.03, respectively. Variations in the pKa of methyl red and phenol red as functions of wt % cosolvents are discussed in terms of solvent properties.