Second-order rate constants ($k_{HOO^-}$) for the nucleophilic substitution reactions of Y-substituted-phenyl diphenylphosphinates (4a-4i) with $HOO^-$ in $H_2O$ have been measured spectrophotometrically. The ${alpha}$-nucleophile $HOO^-$ is 10-70 times more reactive than the reference nucleophile $OH^-$ although the former is ca. $4pK_a$ units less basic than the latter, indicating the ${alpha}$-effect is operative. The Bronsted-type plot for the reactions of 4a-4i with $HOO^-$ is linear with ${eta}_{lg}=-0.51$, a typical ${eta}_{lg}$ value for reactions which were reported to proceed through a concerted mechanism. The Yukawa-Tsuno plot is also linear with ${ ho}=1.40$ and r = 0.47, indicating that a negative charge develops partially on the O atom of the leaving group, which can be delocalized to the substituent Y through resonance interactions. Thus, the reactions have been proposed to proceed through a concerted mechanism. The magnitude of the ${alpha}$-effect (i.e., the $k_{HOO^-}/k_{HO^-}$ ratio) decreases linearly as the leaving-group basicity increases. It has been concluded that solvation effect is not solely responsible for the ${alpha}$-effect found in this study but the transition-state stabilization through an intramolecular H-bonding interaction is also responsible for the ${alpha}$-effect.