The instant invention relates to a method of phosphorodiamidite production that comprises: (E1) preparing a purified solution of a dialkylamine in a polar solvent as follows:—the dialkylamine dissolved in a polar solvent is contacted with a quantity of phosphorus trihalide that is sufficient to react with the alcohol impurities contained in the dialkylamine but sufficiently low to leave a major part of the dialkylamine unreacted, whereby a mixture is obtained that contains the dialkylamine in the polar solvent and reaction products of the impurities with the phosphorous trihalide;—the unreacted dialkylamine and polar solvent present in the mixture obtained in step (E1.1.) are extracted from the solution S by their difference of volatility, typically by distillation, whereby the purified solution of the dialkylamine in the polar solvent is obtained; (E2) the purified solution of dialkylamine in a polar solvent as obtained in step (E1) is reacted with a phosphorus trihalide, whereby an intermediate compound is formed; (E3) the intermediate compound obtained in step (E2) is reacted with a hydroxyalkyl compound in the presence of a non-polar co-solvent.