Novel phosphate triester derivatives of 3'-acetylthymidine, and of the anti-HIV nucleoside analogue AZT have been prepared by phosphorochloridate chemistry. These materials are designed to act as membrane-soluble pro-drugs of the bio-active free nucleotides. In particular, novel glycolate and lactate phosphate derivatives have been prepared. In vitro evaluation revealed the AZT compounds to have a pronounced and selective antiviral effect, the magnitude of which varied considerably with the nature of the phosphate blocking group.