Amino-PEG24-t-butyl ester is a PEG derivative containing an amino group with a t-butyl protected carboxyl group. The hydrophilic PEG spacer increases solubility in aqueous media. The amino group is reactive with carboxylic acids, activated NHS esters, carbonyls (ketone, aldehyde) etc. The t-butyl protected carboxyl group can be deprotected under acidic conditions.