The rate of the solvent substitution reaction, CoCl 3 S ร +Cl ร YCoCl 4 2ร +S (S-solvent) has been studied by means of stopped flow spectrophotometry in dimethyl sulfoxide (DMSO) and N,N-dimethylformamide (DMF), and the rate constant k f , and the activation enthalpy D f H p and entropy D f S p have been determined. The negative and large D f S p values of ร38 and ร68 J K ร1 mol ร1 in DMSO and DMF, respectively, evidently indicate that an associative mechanism operates or the five-coordinate species CoCl 4 S 2ร forms at the transition state. The corresponding D f H p values are 48 and 36 kJ mol ร1 in DMSO and DMF, respectively. To discuss solvation enthalpy difference of the CoCl 4 S 2ร at the transition state in the solvents, the transfer enthalpy D t H p of the activated species from DMF to DMSO was evaluated by combining the D f H p values and the enthalpies of transfer D t H8 of CoCl 3 ร and Cl ร from DMF to DMSO. The D t H p value thus obtained is 18 kJ mol ร1 , the value indicating that the solvation enthalpy of the activated species in DMF is more negative than that in DMSO despite that the electron-pair donating ability of DMF is weaker than that of DMSO. This implies that the entering chloride ion suffers from an appreciable steric hindrance of DMSO at the transition state.