Dissociation constants (Ka) of an indicator acid, 2,5-dinitrophenol in 20, 40, 50, 60 and 70 wt.% ethanol-water mixtures at 25±0.50C temperature were determined spectrophotometrically. The pure indicator acid and potassium salt of indicator acid show maxima at 450 nm in the visible range. At first the spectrophotometric data have been analyzed in terms of an ion-pair intermediate model. But the experimental values of ├É┬?In- potassium salt of indicator acid and the derived values of intercept of ├É┬?In- using ion-pair intermediate model show a sharp difference. So this model was not applicable in this case. For this an alternative treatment, simple dissociation model has been applied. The maximum drop in pKa was found to occur around 20 wt.% ethanol-water mixtures. This has been explained as due to increase in solvation energy for anion of the acid by London Dispersion Force Mechanism. The dramatic increase in pKa values above 20 wt.% ethanol has been assumed to be due to decrease in dielectric constant of the medium.