[en] Second-order rate constants (k_N) have been measured for nucleophilic substitution reactions of S-4-nitrophenyl thiobenzoate with a series of alicyclic secondary amines in H₂O containing 20 mol % DMSO at 25.0 ± 0.1 .deg. C. The Brφnsted-type plot exhibits a downward curvature, i.e., β_nuc decreases from 0.94 to 0.34 as the amine basicity increases. The reactions in the aqueous DMSO have also been suggested to proceed through a zwitterionic tetrahedral intermediate (T^±) with change in the RDS on the basis of the curved Brφnsted-type plot. The reactions in the aqueous DMSO exhibit larger k_N values than those in the aqueous EtOH. The macroscopic rate constants (k_N) for the reactions in the two solvent systems have been dissected into the microscopic rate constants (k₁ and k₂/k_-1 ratio) to investigate effect of medium on reactivity in the microscopic level. It has been found that the k₂/k_-1 ratios are similar for the reactions in the two solvent systems, while k₁ values are larger for the reactions in 20 mol % DMSO than for those in 44 wt % EtOH, indicating that the larger k₁ is mainly responsible for the larger k_N. It has been suggested that the transition state is more stabilized in 20 mol % DMSO through mutual polarizability interaction than in 44 wt % EtOH through H-bonding interaction