Abstract: The results of using gas chromatography to follow the reactions revealed that four different reactions were existed simultaneously in the isomerization of O-alkyl phosphorodichloridothioates. They were considered as a combination of rearrangement, decomposition and disproportion reaction as well as hydrolysis. The reaction mechanism was preliminarily discussed according to HSABtheory. It was the first time that S-isopropyl phosphorodichloridothioate was synthesized by using the isomerization method.