Hydroxy-PEG2-PFP ester is a PEG derivative containing a hydroxyl group with a PFP ester group. It can be used to label the primary amines (-NH2) of proteins, amine-modified oligonucleotides, and other amine-containing molecules. PFP esters have similar applications as the NHS esters, but are more stable in aqueous solution. The hydroxyl group enables further derivatization or replacement with other reactive functional groups.