Second-order rate constants for the reactions of phenyl Y-substituted-phenyl carbonates 5a-g with Z-substituted-phenoxides ($k_{Z-PhO^-}$) have been measured spectrophotometrically in 80 mol % $H_2O$/20 mol % DMSO at $25.0{pm}0.1^{circ}C$. 4-Nitrophenyl phenyl carbonate (5e) is up to 235 times more reactive than 4-nitrophenyl benzoate (4e). The Br$o$nsted-type plot for the reactions of 5e with Z-substituted-phenoxides is linear with ${eta}_{nuc}=0.54$, which is typical for reactions reported previously to proceed through a concerted mechanism. Hammett plots correlated with ${sigma}^o$ and ${sigma}^-$ constants for the reactions of 5a-f with 4-chlorophenoxide exhibit highly scattered points. In contrast, the Yukawa-Tsuno plot results in an excellent linear correlation with ${ ho}_Y=1.51$ and r = 0.52, indicating that the leaving-group departure occurs at the rate-determining step (RDS). A stepwise mechanism, in which leaving-group departure occurs at RDS, has been excluded since the incoming 4-$ClPhO^-$ is more basic and a poorer nucleofuge than the leaving Y-substituted-phenoxides. Thus, the reaction has been concluded to proceed through a concerted mechanism. Our study has shown that the modification of the nonleaving group from benzoyl to phenyloxycarbonyl causes a change in the reaction mechanism (i.e., from a stepwise mechanism to a concerted pathway) as well as an increase in the reactivity.