A process for the preparation of a poly(alkoxylated) oligonucleotide is provided. The process comprises reacting an oligonucleotide which has been purified by ultrafiltration with a poly(alkoxide) thereby to form a poly(alkoxylated) oligonucleotide. The poly(alkoxyalted) oligonucleotide may be separated from non-poly(alkoxyalted) oligonucleotide by ultrafiltration under denaturing conditions, such as the presence of organic solvents, for example, ethanol; the presence of urea; the addition of chaotropic salts, for example perchlorate and guanidinium salts; the presence of formamide; and the application of heat, for example a temperature of up to about 70.degree. C.