A highly efficient method for the preparation of LNA (Locked Nucleic Acid) phosphoramidite monomers with 2-cyanoethyl-N,N,N′,N′-tetraisopropylphosphorodiamidite and 4,5-dicyanoimidazole has been devised. The quality of the phosphoramidites prepared in this manner is equal to HPLC purified phosphoramidites and can easily be used for oligonucleotide synthesis without further purification. In addition the possibility of using 4,5-dicyanoimidazole in catalytic amounts has been investigated and showed optimum results when 0.7 equivalent was used, and that reducing the amount further leads to undesired phosphitylation of the nucleobase. Furthermore it is demonstrated that LNA phosphoramidite monomers are exceedingly stable in acetonitrile solution thereby prolonging the effective lifetime of the reagent significantly.