For reactions involving electron transfer or nucleophilic attack on the transition state/excited state of metal complex in aquo-organic solvent mixtures, a linear relationship between logarithms of rate constant and solvent empirical parameters can be derived. The relationship, Y-S = Y-0 + a(1)X(1) + a(2)X(2) +... + a(n)X(n), fits well for both the measured quantity in a solvent and solvent dependent property. From the quantitative estimation of the coefficients, the dominant solvent stoichiometric and other substantial medium effects on reactivities can be confirmed. Electron-transfer reaction of Co(NH3)(4)ox(+) complex with Fe(CN)(6)(4-) in six different water-cosolvent binary mixtures have been investigated (cosolvent: methanol or 1,4-dioxane 5-30% (v/v)) and the respective k(et) and K-IP values have been experimentally determined. The k(et) values evaluated in mixed solvents show linearity with x(org) and is an element of(r)(-1), reflecting the individuality of the cosolvent and thereby suggesting that the relative stability of the ion pair is governed by the preferential solvation effect. The thermodynamic parameters (DeltaH(#), DeltaS(#), and DeltaG(#)) are sensitive to the structural changes in the bulk solvent phase. Likewise, irradiation (at 254 nm) of the charge-transfer bands of Co(NH3)(4)ox(+) in varying compositions of water-methanol/1,4-dioxane mixtures results in reduction of Co(III) to Co(II); with respect to x(org), (phi(Co(II)) increases up to 1.6-fold (organic cosolvent 5-30% (v/v)). The predominance of Co(II) formation at higher x(org) is due to solvent-assisted reduction in mixture solvents. There is a linear relationship between phi(Co(II)) with is an element of(r)(-1) of the medium indicating the solvational contributions. The experimentally determined k(et) and phi(Co(II)) are well correlated through multiple regression equations based on proposed models to understand the consequences of short range and long-range solvation effects. A quantitative estimation of relative importance of the components is attempted. A number of empirical solvent parameters are used in the multiple regression equations. The correlation analysis reveals significant information on the effect of solvent-solvent and solvent-solute interactions on reactivities. The investigation is a novel attempt to understand the "solvent tuning" of reactions and to obtain a quantitative measure of the relative importance of the influences of solvents.