Abstract: The patent describes homo- or hetero-bifunctional derivatives of polyethylene glycol (PEG) obtained by selectively modifying the hydroxyl end groups of commercial PEGs of different molecular weights with reactive bifunctional molecules. The synthesis process comprises steps involving sequential derivatization of PEG terminal hydroxyl groups with a bifunctional compound, purification of the bifunctional PEG obtained and/or protection of its functional groups with possible selective removal of terminal protections. The bifunctional PEGs (biPEG) obtained are usable as carriers and/or stabilizers for substances used for diagnostic, prophylactic and therapeutic purposes, conjugated to the same biocompatible biPEG support. The bifunctionalized PEG derivatives can also be used in supported synthesis processes.