Argentinated (silver-containing) oligopeptides fragment under low-energy collision conditions to yield abundant argentinated product ions. The structures of the [b2 − H + Ag]+ and [a2 − H + Ag]+ ions have been determined by means of tandem mass spectrometry and confirmed by comparison with synthesized derivatives of the candidate [b2 − H + Ag]+ ion structure. The [b2 − H + Ag]+ ion was found to be an N-argentinated oxazolone, which could subsequently form the [a2 − H + Ag]+ ion and other product ions after collision activation. Tripeptides containing proline as their second residue were observed to form a relatively less abundant [b2 − H + Ag]+ ion, which was postulated to be an argentinated ketene.