Ald-Ph-PEG5-t-butyl ester is a hydrophilic PEG derivative with a benzaldehyde moiety and a t-butyl ester. The benzaldehyde can react with primary amine groups. The t-butyl ester is an acid labile protecting group. Upon removal the carboxylic acid can be reacted with primary amines. The hydrophilic PEG linker increases the water solubility of a compound in aqueous media.