Earlier work on the hydrolysis of aryl phosphinothioate esters has led to contradictory mechanistic conclusions. To resolve this mechanistic ambiguity, we have measured linear free energy relationships (βnuc and βlg) and kinetic isotope effects for the reactions of oxyanions with aryl dimethylphosphinothioates. For the attack of nucleophiles on 4-nitrophenyl dimethylphosphinothioate, βnuc = 0.47 ± 0.05 for phenoxide nucleophiles (pKa < 11) and βnuc = 0.08 ± 0.01 for hydroxide and alkoxide nucleophiles (pKa ≥ 11). Linearity of the plot in the range that straddles the pKa of the leaving group (4-nitrophenoxide, pKa 7.14) is indicative of a concerted mechanism. The much lower value of βnuc for the more basic nucleophiles reveals the importance of a desolvation step prior to rate-limiting nucleophilic attack. The reactions of a series of substituted aryl dimethylphosphinothioate esters give the same value of βlg with the nucleophiles HO- (β = −0.54 ± 0.03) and PhO- (β = −0.52 ± 0.09). A significantly better Hammett correlation is obtained with σ- than with σ or σ°, as expected for a transition state involving rate-limiting cleavage of the P−OAr bond. The 18O KIE at the position of bond fission (18k = 1.0124 ± 0.0008) indicates the P−O bond is ∼40% broken, and the 15N KIE in the leaving group (15k = 1.0009 ± 0.0003) reveals the nucleofuge carries about a third of a negative charge in the transition state. Thus, both the LFER and KIE data are consistent with a concerted reaction and disfavor a stepwise mechanism.