Second-order rate constants (<TEX>$k_N$</TEX>) have been measured spectrophotometrically for the nucleophilic displacement reactions of 4-nitrophenyl X-substituted-cinnamates (4a-4e) and Y-substituted-phenyl cinnamates (5a-5e) with Z-substituted-phenoxide anions in 80 mol % <TEX>$H_2O$</TEX>/20 mol % DMSO at <TEX>$25.0{\pm}0.1^{\circ}C$</TEX>. The Hammett plot for the reactions of 4a-4e with 4-chlorophenoxide (4-<TEX>$ClPhO^-$</TEX>) consists of two intersecting straight lines, which might be taken as a change in the rate-determining step (RDS). However, it has been concluded that the nonlinear Hammett plot is not due to a change in the RDS but is caused by stabilization of the ground state of substrates possessing an electron-withdrawing group in the cinnamoyl moiety through resonance interactions, since the Yukawa-Tsuno plot exhibits an excellent linear correlation with <TEX>${\rho}X=0.89$</TEX> and r = 0.58. The Br<TEX>${\o}$</TEX>nsted-type plot for the reactions of 4-nitrophenyl cinnamate (4c) with Z-substituted-phenoxides is linear with <TEX>${\beta}_{nuc}=0.76$</TEX>. The Br<TEX>${\o}$</TEX>nsted-type plot for the reactions of Y-substituted-phenyl cinnamates (5a-5d) with 4-chlorophenoxides (4-<TEX>$ClPhO^-$</TEX>) is also linear with <TEX>${\beta}_{lg}=-0.72$</TEX>. The Hammett plot correlated with <TEX>${\sigma}^-$</TEX> constants for the reactions of 5a-5d results in a much better linear correlation than that correlated with <TEX>${\sigma}^o$</TEX> constants, indicating that a partial negative charge develops on the O atom of the leaving aryloxide. Thus, the reactions have been concluded to proceed through a concerted mechanism.