A site-selective dual-functionalization of peptides is presented, involving readily available maleimides as well as N-hydroxylamines. The modification proceeds through a three component 1,3-dipolar cycloaddition, forming a stable product. This was exemplified by the one-pot attachment of two molecular imaging moieties to a tumor binding cyclic peptide, and was extended to the conjugation of a DOTA chelator to a 12 kDa protein.