It has previously been shown that collagen is degraded at room temperature to water-soluble products by dilute solutions of hydrogen peroxide to which very low concentrations of transition metal ions have been added. The present study is an attempt to determine the locations along the collagen chains that are being attacked in this reaction, using lower molecular weight peptides as model compounds. For comparison, the effect on the peptides of a dilute solution of hydrogen peroxide containing no added metal ions was also investigated. The results show that dipeptides are split at the peptide bond by hydrogen peroxide. Ammonia is major product. The rate of attack varies with the nature of the amino acids forming the peptide bond, and also, for the same two amino acids, with the relative positions (N-terminal or C-terminal) of the amino acids. Ammonia is produced more readily if the hydrogen peroxide contains a trace of cupric ion. Studies with one tripeptide (gamma-L-glutamyl-L-cysteinylglycine) also show splitting of the peptide bond by the cupric ion-hydrogen peroxide system. The poly(amino acid), polyproline I (molecular weight approximately 14, 000), is attacked by dilute hydrogen peroxide, with the splitting, under the reaction conditions used, of approximately 20 of some 140 peptide bonds present in the molecule. Implications of these facts in the oxidative degradation of collagen are discussed.