A method of phosphorodiamidite production that comprises the following steps: a purified solution of the dialkyl amine in a polar solvent is prepared as follows: (E1.1.) 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, and optionally with other impurities contained in the dialkylamine or in the polar solvant, 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; (E1.2.) 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 dialkyl amine in the polar solvent is obtained; the purified solution of dialkyl amine in a polar solvent as obtained in step is reacted with a phosphorus trihalide is whereby an intermediate compound is formed; the intermediate compound obtained in step is reacted with a hydroxyalkyl compound in the presence of a non-polar co-solvent. a purified solution of the dialkyl amine in a polar solvent is prepared as follows: (E1.1.) 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, and optionally with other impurities contained in the dialkylamine or in the polar solvant, 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; (E1.2.) 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 dialkyl amine in the polar solvent is obtained; (E1.1.) 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, and optionally with other impurities contained in the dialkylamine or in the polar solvant, 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; (E1.2.) 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 dialkyl amine in the polar solvent is obtained; the purified solution of dialkyl amine in a polar solvent as obtained in step is reacted with a phosphorus trihalide is whereby an intermediate compound is formed; the intermediate compound obtained in step is reacted with a hydroxyalkyl compound in the presence of a non-polar co-solvent.