The occurrence of the reverse PO - PS reaction is of interest in view of its potential utility for the conversion of phosphoryl compounds into corresponding phosphorothioyl analogues. Although such a synthetic aspect was not evaluated in the current study, the optimization of the reaction conditions and carboxylic thioanhydride or thiocarboxylate/anhydride sytem might produce a new mild reagent for the PO - PS transformation.The stereochemical results of oxidation of 1 and 2 are explained by the racemization of the sulfide, selenide, or oxide due to the multiple occurrence of the reversible processes shown in Scheme I11 (each occurring with inversion at phosphorus) before the inactivation of the trifluorothioacetate can take place. The relatively higher degree of inversion observed in oxidations of triesters 3 and 4 can be explained by a much lower nucleophilicity of selenium atom in these cases as compared to phosphine selenide 2 (due to electronic effect of oxygen substituents at phosphorus), slowing down the rates of formation of 12 and 13 and lowering the probability of a return to phosphoroselenonates. In fact, the low-temperature 31P NMR spectra of O,O,O-trimethyl phosphoroselenoate/TFAA did not indicate the formation of such intermediates at concentrations above the detection limit.