Kinetic studies on the reactions of O-methyl (2) and O-ethyl (3) phenyl phosphonochloridothioates with X-pyridines have been carried out in acetonitrile at 35.0°C. The pyridinolysis rates of 2 are slightly faster than those of 3. The substituent effects of X on the pyridinolysis rates of 2 and 3 are similar. The Hammett and Brönsted plots with X in the nucleophiles are biphasic concave upward with a break point at X = 3-Ph for both substrates. The stepwise mechanism with rate-limiting leaving group departure from the intermediate is proposed based on the βX values and biphasic concave upward free energy relationship for both substrates. The biphasic concave upward free energy relationships are rationalized by a frontside nucleophilic attack TSf with more basic pyridines and backside attack TSb with less basic pyridines for both substrates.