New phosphorylated monomethoxy poly(ethylene glycol)s (MPEGs) were obtained and used to prepare diMPEG-conjugated oligonucleotides. The chemical similarity with other organic molecules carrying diester bonds can be easily envisaged, allowing for further applications of these new MPEG units. Both high-molecular-mass phosphodiester- and phosphoramidite-derivatives have been obtained, characterized and evaluated for their reactivity.