According to the present invention, there is provided a process for the preparation of a first compound selected from peptides, oligonucleotides and peptide nucleic acids. The process comprises synthesizing the first compound and then separating the first compound formed in step (i) from a second compound, which is a reaction by-product of the synthesis of the first compound and/or an excess of a reagent used for the synthesis of a first compound by a process of diafiltration. The membrane used for the diafiltration process is stable in organic solvents and provides a rejection for the first compound which is greater than the rejection for the second compound.