A process for the purification of an oligonucleotide synthon is provided. The process comprises subjecting an organic solution comprising an oligonucleotide synthon and lower molecular weight impurities to nanofiltration whereby the ratio of an oligonucleotide synthon to lower molecular weight impurities in the solution is increased after the nanofiltration. Preferably, the oligonucleotide synthon is a nucleoside phosphoramidite or nucleoside H-phosphonate. The nanofiltration membrane is preferably a polyimide membrane having a molecular weight cut off of 400.