Aminooxy-PEG5-azide is a biolabeling reagent comprising an aminooxy group and an azide group. The hydrophilic PEG spacer increases solubility in aqueous media. The aminooxy group can be used in bioconjugation. by reacting with an aldehyde moiety to form an oxime bond, if a reductant is used, it will form a hydroxylamine linkage. The azide group can react with alkyne, BCN, DBCO via Click Chemistry to yield a stable triazole linkage. Aminooxy compounds are very reactive and sensitive; they cannot be stored for long term. Immediate use (within 1 week) is highly recommended.