A process for the synthesis in solution phase of a phosphorothioate triester is provided. The process comprises the solution phase coupling of an H-phosphonate with an alcohol in the presence of a coupling agent to form an H-phosphonate diester. The H-phosphonate diester is oxidised in situ with a sulfur transfer agent to produce the phosphorothioate triester. Preferably, the H-phosphonate and alcohol are protected nucleosides or oligonucleotides. Oligonucleotide H-phosphonates which can be used in the formation of phosphorothioate triesters are also provided.