Second-order rate constants ($k_{CN^-}$) have been measured for nucleophilic substitution reactions of Y-substituted phenyl benzoates (1a-r) with $CN^-$ ion in 80 mol % $H_2O$/20 mol % DMSO at $25.0{pm}0.1^{circ}C$. The Br${o}$nsted-type plot is linear with ${eta}_{1g}$ = -0.49, a typical ${eta}_{1g}$ value for reactions reported to proceed through a concerted mechanism. Hammett plots correlated with ${sigma}^{circ}$ and ${sigma}^-$ constants exhibit many scattered points. In contrast, the Yukawa-Tsuno plot for the same reaction exhibits excellent linearity with ${ ho}_Y$ = 1.37 and r = 0.34, indicating that a negative charge develops partially on the oxygen atom of the leaving aryloxide in the rate-determining step (RDS). Although two different mechanisms are plausible (i.e., a concerted mechanism and a stepwise pathway in which expulsion of the leaving group occurs at the RDS), the reaction has been concluded to proceed through a concerted mechanism on the basis of the magnitude of ${eta}_{1g}$ and ${ ho}_Y$ values.