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Kinetic studies on the reactions of Y-aryl phenyl isothiocyanophosphates with substituted X-anilines and deuterated X-anilines were carried out in acetonitrile at 55.0 °C. The free-energy relationships with X in the nucleophiles were biphasic concave upwards with a break region between X = H and 4-Cl, giving unusual positive ρX and negative βX values with less basic anilines (X = 4-Cl and 3-Cl). A stepwise mechanism with rate-limiting bond breaking for more basic anilines and with rate-limiting bond formation for less basic anilines is proposed based on the positive and negative ρXY values, respectively. The deuterium kinetic isotope effects involving deuterated anilines (XC6H4ND2) showed primary normal and secondary inverse DKIEs for more basic and less basic anilines, rationalized by frontside attack involving hydrogen-bonded four-center-type TSf and backside attack TSb, respectively. The positive ρX values with less basic anilines are substantiated by the tight TS, in which the extent of the bond formation is great and the degree of the bond breaking is considerably small.