(4,4'-Dimethoxytrityl)-polyethylene glycol-(2-cyanoethyl-N,N-diisopropyl)-phosphoramidites and (4,4'-dimethoxytrityl)-polyethylene glycol derivatized glass supports were synthesized and used to introduce PEG with polymerization degrees from about 6 to more than 100 at 3'- and 5'-ends of oligonucleotides by means of automated synthesis. The electrophoretic mobility of the conjugates is altered with respect to the unmodified oligonucleotides. The hydrophobicity of the conjugates increases with increasing length of the PEG chain.