A process for the synthesis of an oligonucleotide is provided in which an oligonucleotide is assembled on a swellable solid support using the phosphoramidite approach in the presence of an activator, wherein the activator is not tetrazole or a substituted tetrazole. Preferred activators are pyridinium, imidazolinium and benzimidazolinium salts; benzotriazole and derivatives thereof; and saccharin or a saccharin derivative. Preferred swellable solid supports comprise functionalised polystyrene, partially hydrolysed polyvinylacetate or poly(acrylamide).