Abstract 2-Iodoethyl (methyl α- d-glucopyranosid)uronate and 2-iodoethyl (methyl β- d-glucopyranosid)thiouronate were prepared in five steps by an efficient synthetic route starting from d-glucuronic acid. Both compounds were used to alkylate dithymidine phosphorothioate and phosphorodithioate diesters, leading to the corresponding phosphotriesters 12 to 15. Hydrolytic stability of 12–15 was studied in different biological media. The enzymatic hydrolysis of 12–15 was accompanied by another reaction resulting in formation of the dithymidine phosphodiesters. Several possible mechanisms for these reactions are proposed.