A method of phosphorodiamidite production comprising the steps of reacting a phosphorus trihalide with a dialkyl amine in a polar solvent to form an intermediate compound. This intermediate compound is then subsequently reacted with an hydroxylalkyl compound and a dialkyl amine in the presence of a non-polar cosolvent. Following filtration to remove the solid by-product the two solvents form separate layers. This is advantageous as the upper, non-polar solvent, layer contains the high-purity phosphorodiamidite product.