Abstract: The absolute configurations of O-ethyl O-phenyl phosphorothioic acid ( 1) and its thiol-esters (2) were established by the following reactions (Scheme l.):the chlorination of (-)-(2)and esterification of (-)-O-ethyl O-phenyl phosphorochloride; (3) It has been confirmed that the chlorination of (-)-(2) either with Cl₂ or SO₂ Cl₂ proceeds with retention of the configuration, and the esterification of(-)-(3) forms (-)-O-ethyl O-methyl O-phenyl phosphate; (4) proceeding with complete inversion of its configuration..a.R′=CH₃ b.R′=n-pr c.R′=C₂H₂OET Since the absolute configurations of (-)-(4) are kno wn as: (S)-(-)-(4), (R)-( + )-(4), the absolute configurations of ( + )-(1) and (-)-(2 ) should be (R)-( + )-(l) and (R)-(-)-(2),respectively. This result consists with that one obtained by X-ray diffraction method.