In this work infinite dilution activity coefficients (gamma(i)nfinity) of 2-chloro-2-methylpropane (tert-BuCl) in solvent mixtures of methanol/ethane-1,2-diol, ethanol/ethane-1,2-diol, and 2-methoxyethanol/ethane-1,2-diol have been calculated using the UNIFAC group-contribution model. For each system nine different compositions of the solvent mixtures were considered. The modification of the Flory-Huggins term in the combinatorial equation and the group-contribution parameters from the vapor-liquid equilibrium parameter table were used. These values together with the Gibbs energies of activation (Delta(double dagger) G), obtained from the new experimental rate constant values of the solvolytic reactions of the same substrate in the binary alcoholic mixtures, allow the analysis of the solvent effect during the activation process, by means of the contributions of the initial state and the transition state.