The solvolysis rate constants (k(obs)) of tert-butyl chloride are measured in 20% (v/v) 2-PrOH-H2O mixture at 15 temperatures ranging from 0 to 39 degreesC. Examination of the temperature dependence of the rate constants by the weighted least squares fitting to two to four terms equations has led to the three-term form. In k(obs) = a(1) + a(2)T(-1) + a(3) ln T, as the best expression. The activation parameters, DeltaH(double dagger) and DeltaS(double dagger), calculated by using three constants a(1), a(2) and a(3) revealed the steady decrease of approximate to 1 kJ mol(-1) per degree and 3.5 J K-1 mol(-1) per degree, respectively, as the temperature rises. The sign change of DeltaS(double dagger) at approximate to 20.0 degreesC and the large negative heat capacity of activation, DeltaC(p)(double dagger) = - 1020 J K-1 mol(-1), derived are interpreted to indicate an S(N)1 mechanism and a net change from water structure breaking to electrostrictive solvation due to the partially ionic transition state. Confidence intervals estimated by the Monte Carlo method are far more precise than those by the conventional method. (C) 2004 Elsevier B.V. All rights reserved.