Oligonucleotides having bioreversible phosphate blocking groups can be used as pro drugs for the oligonucleotides. The bio-reversible blocking groups can be removed by intercellular enzymes, particularly carboxyesterases, to provide the unprotected oligonucleotide. The oligonucleotides can be prepared via post synthesis alkylation reactions or utilizing amidite type chemistry wherein the bioreversible phosphorus protecting group is formed as an integral part of the amidite reagent.