Second-order rate constants have been measured for alkaline hydrolysis of O-aryl thionobenzoates (X-C6H4-CS-OC6H 4-Y) in 80 mol % H2O-20 mol % DMSO at 25.0 ± 0.1 °C. The Hammett plot for the reaction of O-4-nitrophenyl X-substituted thionobenzoates (X-C6H4-CS-OC6H 4-NO2, 1a-e) exhibits a downward curvature. However, a possible traditional explanation in terms of a change in the rate-determining step (RDS) has been considered but rejected. The proposed explanation involves stabilization of the ground-state (GS) through-resonance interaction between the electron-donating substituent X and the thionocarbonyl functionality on the basis of the linear Yukawa-Tsuno plot obtained for the same reaction. The Brønsted-type plot for the reaction of O-aryl thionobenzoates (C 6H5-CS-OC6H4-Y, 2a-i) is linear but exhibits many scattered points with a small βlg, (-0.35). The Hammett plot for the same reaction shows rather poor correlation with σ- constants but much better correlation with σ° constants. The alkaline hydrolysis of O-aryl thionobenzoates (1a-e and 2a-i) has been proposed to proceed through an addition intermediate in which bond formation is the RDS.