2-Phenoxy- and 2-dialkylamino-2-oxa-1,3,2-oxathiaphospholanes react with PCl5 with ring opening and formation of the corresponding acid chlorides of thiophosphoric acid. Under analogous conditions, 2-phenoxy-2-oxo-1,3,2-oxathiaphosphorinanes do not react with PCl5, while acid chlorides of thiophosphoric acid are formed in low yield along with other products in the reaction of 2-dialkylamino-2-oxo-1,3,2-oxthiaphosphorinanes with PCl5.