Physical Chemistry Laboratories, Jadavpur University, Calcutta-700 032 Manuscript received 13 April 1993 Kinetic solvent effects (KSE) on a key SN1 type reaction, such as hydrolysis/solvolysis of t-butylchloride (t-BuCI) in aqueous mixtures of some protic cosolvents, viz. methoxyethanol (ME), ethylene glycol (EG) and glycerol (GL), aprotic cosolvents, viz. 1,2-dimethoxyethane (DME) and dioxane (D) and dipolar aprotic cosolvents, like N,N-dimethylformamide (DMF) and dimethylsulphoxide (DMSO), have been analysed in the light of solvation energetics of the initial state (IS) and transition state (TS). The required rate constants (ks) at different temperatures have been determined earliar by conductometric method and the relevant activation parameters by use of the coefficients of the rate constant-temperature equation : log ks = AT-1 + B log T + C. Transfer energetics of IS and TS from water (w) to other solvents (s) were evaluated by coupling the activation energetics (��H��) with the previously determined enthalpies of solution (\(��H_N^o\)) of IS : t-BuCl and by coupling the activation free energies (��G��) with the solvation free energies of the ion-pair (CH3)3NH+Cl- ��� a newly proposed model of the TS : (CH3)3C��+...Cl��-, as obtained after due correction of the previously determined solvation free energies of the salt (CH3)3NHCI in the solvents by an indirect method. Each of the transfer energetic terms of IS and TS was dissected into cavity, dipole-dipole, dispersion and the rest, hydrophobic-hydrophilic (HH) hydration terms. The latter is the composite contributions of hydrophobic hydration (HbH) arising from 3(CH) groups of partially charged (CH3)3C��-group and the hydrophilic hydration (H1H) arising from partially charged CI��- atom of the TS : (CH3)3C�� +...C��1- of t-BuCI. ,HH(i)��� and particularly ,HH(i)���icomposition profiles (X = G, H and S and i = IS or TS) were then analysed in terms of four-step transfer process of Kundu et al. and the involved physicochemical properties and especially the relative structuredness of solvents, which guide the HH hydration terms. This helped to understand the observed unique minima in T��S����� and hance ��S�����composition profiles in these and other aquo-organic solvents, which proved baffling since the days of the classical work of Ingold and Hughes, so to say on this key SN1-type reaction