The nucleophilic substitution reactions of diisopropyl chlorothiophosphate (5) with X-pyridines have been kinetically studied in MeCN at $35.0^{circ}C$. The Hammett and Br$ddot{o}$nsted plots for the substituent X variations in the nucleophiles show biphasic concave upwards with a break point at X = 3-Ph. The pyridinolysis rate of 5 exhibits great negative deviation from the Taft plot. A concerted $S_N2$ mechanism is proposed involving a change of the attacking direction of the X-pyridines from a frontside attack with the strongly basic pyridines to a backside attack with the weakly basic pyridines.