Abstract: The chlorination of (+)-O-ethyl O-phenyl phosphorothioic acid (1R) with PC₁₅ gave (-)-O-ethyl O-phenyl phosphorothiochloride (5R) and proceeded with inversion of configuration. lR, (-)-O-ethyl O-phenyl O-methyl phosphorothionate (6R) and (+)- or (-)-O-ethyl O-phenyl N-ethyl(or N, N-diethyl) phosphorothioamide (7R) were obtained with inversion of configuration at Patom from the hydrolysis, alcoholysis and aminolysis of 5R, respec tively. Acid-catalysed hydrolysis and alcoholysis of 7Rformed to (-)-O-ethyl O-phenyl phosphorothioic acid (lS) and (+)-O-ethyl O-phenyl O-methyl phosphorothionate(6S) respectively were realized, while both reactions proceeded with inversion. The oxidation of 6R with either H₂O₂ or m-chloroperoxybenzoic acid resulted in (-)-O-ethyl O-methyl O-phenyl phosphoronate (4S) in the same stereochemi-cal course with retention of configuration at P atom, but it was discovered that, by using dimethyl sulphoxide-iodine,6R was isomerized to (-) -O-ethyl O-phenyl S-methyl phosphorothiolate(2R), instead of oxidation to 4S.