The second-order rate constants ($k_{OH^-}$) for the reactions of Y-substituted-phenyl diphenylphosphinates (4a-4i) with $OH^-$ in $H_2O$ at $25.0{pm}0.1^{circ}C$ have been measured spectrophotometrically. Comparison of $k_{OH^-}$ with $k_{EtO^-}$ (the second-order rate constants for the corresponding reactions with $EtO^-$ in ethanol) has revealed that $EtO^-$ is less reactive than $OH^-$ although the former is ca. 3.4 $pK_a$ units more basic than the latter, indicating that the reactivity of these nucleophiles is not governed by their basicity alone. The Br${o}$nsted-type plot for the reactions of 4a-4i with $OH^-$ is linear with ${eta}_{lg}$ = -0.36. The Hammett plot correlated with ${sigma}^-$ constants results in a slightly better correlation than that correlated with ${sigma}^{circ}$ constants but exhibits many scattered points. In contrast, the Yukawa-Tsuno plot for the same reactions exhibits an excellent linear correlation with ${ ho}$ = 0.95 and r = 0.55. The r value of 0.55 implies that a negative charge develops partially on the O atom of the leaving group. Thus, the reactions of 4a-4i with $OH^-$ have been concluded to proceed through a concerted mechanism.