Diethyl acetal-PEG-OH (Acetal-PEG-OH) is a linear heterobifunctional PEG reagent with an acetal and OH group. It is a useful reagent for bioconjugation with a PEG spacer. Acetal can be easily reduced to aldehyde under acidic condition (pH 4.0). Acetal-PEG-OH may be used to introduce aldehyde in PEG copolymers. The aldehyde group can react with amine and thiol group for bioconjugation.