1H NMR is used to monitor the hydrolysis of methionine- and cysteine-containing peptides, promoted by palladium (II) diaqua complexes. The cleavage reaction is somewhat sequence-selective. Comparing the hydrolytic rales of Met-aa bond promoted by different palladium (II) complexes, it is found that they have the following sequence: trans-[Pd(Py)2(H2O)2]2 + > cis-[Pd(en)(H2O)2]2+ >>cis-[Pd( N, S-MetH) (H2O)2]2+ . Based on 1H NMR, electro-spray mass spectrometry and kinetic data, a new mononuclear active complex for cleavage is proposed. In the active form, palladium (II) coordinates to substrates through thioether, deprotonated amide nitrogen and amide oxygen of methionyl residue, forming a fused six-membered and five-membered complex, and the Met-aa bond in it is cleaved by external attack of solvent water. It is the first time to find that the Cys-Pro bond is fast cleaved with the half-lives of 50 min at 40℃ . The new finding may have practical application in site-specific cleavage of Cys-Pro containi