Abstract: A process for the preparation of phosphine oxides or sulphides of the formula: ##EQU1## in which R is a linear or branched aliphatic radical containing 5-12 carbon atoms, aralkyl radical selected from the group consisting of lower alkyl of 2-3 carbon atoms substituted by a phenyl group or R is cyclohexyl and X is an oxygen or sulphur atom comprising reacting a halo-compound of the formula R--Hal, in which Hal is chlorine, bromine or iodine, with magnesium to form a Grignard derivative of the formula RMgHal, the reaction being carried out initially in a very small amount of diethyl ether and then in a solvent mixture containing a preponderant amount of an aliphatic or aromatic hydrocarbon, which is liquid at room temperature or triethylamine, adding phosphorus oxychloride or phosphorus sulphochloride to the reaction mixture containing the Grignard derivative, and isolating the phosphine oxide or phosphine sulphide formed by removing the solvent by steam stripping or steam distillation.