Visible spectroscopy and the rotating ring-disk electrode were used to study Cu(II)-peptide complexation of 21 peptides including Nacylated and N-pyroglutamyl peptides. All of the investigated peptides formed a complex with copper ion, and those complexes were electroactive. Peptides containing proline near the amine terminus, and N-acylated peptides, had lower rate constants for the formation of the electroactive complexes. Reaction rates were not signi®cantly decreased in peptides with a pyroglutamyl amine terminus. These complexes demonstrated very low oxidation potentials and some unusual spectroscopic properties.