A process for the preparation of an oligonucleotide having at least one phosphate internucleotide linkage I provided. The process comprises the steps of: a) forming a nascent oligonucleotide comprising a phosphorus (III) internucleotide linkage; and b) oxidation of the nascent oligonucleotide with aqueous iodine solution thereby to form a phosphorus (V) internucleotide linkage; wherein the oxidation is carried out in the presence of a base, the conjugate acid of said base having a pKa higher than the pKa of the conjugate acid of pyridine. Preferably the base is N-methylimidazole.