The origin of the α-effect has been probed through a combination of calorimetric and kinetic studies involving butane-2,3-dione monoximate as α- nucleophile and p-chlorophenoxide as normal nucleophile in the reaction with p-nitrophenyl acetate in DMSO-H2O mixtures, which has been shown to exhibit a bell-shaped profile in the α-effect with solvent composition. The study, involving determination of enthalpies of solution and activation parameters, has allowed a dissection of contributions to the α-effect of ground-state destabilization and transition-state stabilization in these DMSO-H2O solvent media. It has been found that over the solvent composition 0-50 mol % DMSO desolvation of the α-nucleophile is the main driving factor to the increasing α-effect. However, in solvent mixtures covering 50-90 mol % DMSO the thermodynamic activation parameters suggest an interplay of factors that result in the bell-shaped α-effect profile. Discussion is presented that includes possible medium-dependent nonsynchronicity of nucleophile desolvation and bond formation for the α-nucleophile.