Dechlorination of phenylphosphonothioic di-chloride with magnesium in the presence of 2,3-dimethylbutadiene, benzil, and diethyl disulphide gave 1,2-thiaphosphorin, 1,3,2-dioxaphospholene, and phosphonotrithioate derivatives, respectively, and the formation of these products was explained in terms of the intermediacy of phenylphosphinothioylidene (PhPS).