The Kamlet-Taft π*values of mixtures of water with four organic solvents over the entire range of composition have been estimated by solvatochromlc measurements with a series of carefully selected Indicators. The measured π*value of each Indicator is collinear with the average π* value. We Interpret these results as Indicating that the indicators sense the polarlzablltty/dlpolarlty and not the hydrogen bond acidity of the solvent. Examination of relationships betweenπ*and e and between ETand π* and excess properties of π#, ET, and e with respect to volume fraction of the organic cosolvent leads to the conclusion that the principal effect of changing the solvent composition on the observed π* values operates through the dielectric properties of the local medium about the solute. This also supports the previous observation that solvatochromlc shifts of the Indicators used In this study are not very sensitive to solvent hydrogen bond acidity. The measured π*values together with literature ETvalues were used to estimate a values, the hydrogen bond acidity parameter, of aqueous organic solvents.