The formation of a synthetic ferryl-peptide conjugate and mechanistic studies that elucidate its mode of decomposition are presented. A ferry! species is generated from a ligand-dipeptide conjugate 4. The ferryl species [Fe-IV(4)(O)](2+), noted as compound 5, was characterized by UV-vis spectroscopy and by high-resolution electrospray mass spectrometry. The ferryl-peptide conjugate 5 is stable for over 1 h at room temperature. Ester derivatives of 5 decay at different rates, consistent with the remote ester group controlling the stability of the ferryl. The kinetic isotope effect value (4.5) and rho = -1.3 observed with ester derivatives suggest that the mechanism for decomposition of 5 follows a hydrogen-atom-transfer pathway. The formation and decay of 5 was fit to a two-step process, with the decay being unimolecular with respect to the ferryl 5.