In search for protein-modifying catalysts, we have investigated catalytic oxidations of acetyl-protected peptide amide substrates, which resemble amino residues of proteins, using iron-based catalysts and oxidants such as oxone (KHSO₅), H₂O₂, and CH₃CO₃H. Substrates of type Ac-AA-NHtBu (AA=amino acid) were synthesized. Among the iron catalysts investigated, [Fe^II(N₄Py)(MeCN)](ClO₄)]₂ (1) in the presence of excess oxone in aqueous medium was optimal and oxidized the backbone of Ac-Gly-NHtBu and side chains with substrates derived from Phe, Tyr, Trp, and Met. Kinetic data for the reactions of the iron-oxo species [Fe^IV(O)(N₄Py)]²⁺ derived from 1 with the peptide substrates were obtained.