Second-order rate constants (kHOO-) have been measured spectrophotometrically for nucleophilic substitution reactions of Y-substituted phenyl benzenesulfonates (1a-g) with HOO- ion in H2O at 25.0 ± 0.1 °C. The Brønsted-type plot is linear with βlg = -0.73. The Hammett plot correlated with with σ- constants results in much better linearity than σo constants, indicating that expulsion of the leaving group occurs in the rate-determining step (RDS) either in a stepwise mechanism or in a concerted pathway. However, a stepwise mechanism in which departure of the leaving group occurs in the RDS has been excluded since HOO- ion is more basic and a poorer leaving group than the leaving Y-substituted phenoxide ions. Thus, the reactions of 1a-g with HOO- ion have been concluded to proceed through a concerted mechanism. The α-nucleophile HOO- ion is more reactive than its reference nucleophile OH- ion although the former is ca. 4 pKa units less basic than the latter (i.e., the α-effect). TS stabilization through intramolecular H-bonding interaction has been suggested to be irresponsible for the α-effect shown by HOO- ion, since the magnitude of the α-effect is independent of the electronic nature of substituent Y in the leaving group. GS destabilization through desolvation of HOO- ion has been concluded to be responsible for the α-effect found in the this study.