The values of pK a ms (K a ms represents ionization constant of conjugate acid of amine base in mixed waterϪacetonitrile solvent) for all amines, except for charged amine bases, show a mild decrease (ca. 0.1-0.4 pK units) with the increase in CH 3 CN content from 2 to ϳ60% v/v. However, the pK a ms values at 70% v/v CH 3 CN become nearly equal or slightly larger (by Յ0.7 pK units) than the corresponding pK a ms at 2% v/v CH 3 CN for all neutral and charged amines. The values of pK a ms for phenol increase from 10.17 to 13.38 with the increase in the content of CH 3 CN from 2 to 70% v/v in mixed aqueous solvent. Taft reaction constants, *, obtained from the plots of pK a ms against ͚* for primary and secondary amines decrease by ca. 0.8 * units with the increase in the CH 3 CN content from 2 to 70% v/v. The values of pK a ms show an empirical linear relationship with the corresponding values of pK a w (where pK a w represents the pK a obtained in aqueous solvent containing 2% v/v CH 3 CN), which allows the estimation of a pK a in mixed H 2 O-CH 3 CN solvents from that in water.