The Walden Inversion is the only phenomenon connected with optically active compounds for which a possible explanation is not forthcoming on the theories of Pasteur, van't Hoff and Le Bel. Since sign of rotatory power and change of configuration do not necessarily go hand in hand, before it can be decided whether or not a Walden inversion has occurred it is necessary to determine whether the derivative and the parent compound 'possess opposite configurations or not. The causes which bring about such a change in configuration have also to be defected. In the present work, after the resolution by a new method of beta-phenyl -hydoxypropionic acid, the preparation of the p-toluenesulphinate of its d-ethyl ester is described. From a consideration of the reactions of both this eater and the corresponding p-toluene-sulphonate, it is shown that the dextro-rotatory chloro-(or bromo-) ester has the same configuration as the dextro-rotatory hydroxy-ester. Further the p-toluene-sulphinic and the p-toluenesulphonic esters are shown to undergo definite inversion reactions, which probably have a relatively simple mechanism. In the light of the conclusions drawn from the study of the reactions of the toluenesulphiaate the replacement by halogen of the hydroxyl group of the hydroxy-ester by means of thionyl chloride and the halides of phosphorus, both in the presence and in the absence of tertiary bases, has hem examined. From a consideration of the experimental results the conclusions are drawn that the halides of phosphorus always react with inversion of configuration; that thionyl chloride reacts normally in the absence of tertiary bases, but with inversion of configuration in the presence of bases. An interpretation of these results on the basis of the electronic theory of valency is given.