Second-order rate constants have been measured spectrophotometrically for the reactions of substituted phenyl acetates with butane-2,3-dione monoximate and p-chlorophenoxide anions in MeCN-H$_2$O mixtures of varying compositions. The reaction rate, unexpectedly, decreased remarkably upon initial additions of MeCN to H$_2$O up to 30-40 mole ${\%}$ MeCN, and followed by a gradual increase upon further additions of MeCN. The change in solvent composition also influenced the magnitude of the ${alpha}$-effect, i.e., the ${alpha}$-effect increased as the mole ${\%}$ MeCN increased. The solvent dependent ${alpha}$-effect for the present system appears to indicate that the differential solvation between the ${alpha}$-effect nucleophile and the corresponding normal nucleophile is not solely responsible but the difference in the transition-state stabilization is also responsible for the ${alpha}$ -effect in organic solvent-rich region.