Chemical modification of phage libraries has allowed the in vitro evolution of ligands having properties not provided by natural polypeptides. The development of novel and more diverse chemical reactions on phage was hampered by the lack of analytical methods to efficiently monitor the reaction products on the more than 10 000 kDa large filamentous phage particles. Herein, we present a strategy to detect chemically modified peptides on phage based on enzymatic release of peptide from phage and mass spectrometry analysis.