Argentinated (silver-containing) oligopeptides fragment under low-energy collision : conditions to yield abundant argentinated product ions. The structures of the [b(2) - H + Ag](+) and [a(2) - H + Ag](+) ions have been determined by means of tandem mass spectrometry and confirmed by comparison with synthesized derivatives df the candidate [b(2) - H + Ag](+) ion structure. The [b(2) - H + Ag](+) ion was found to be an N-argentinated oxazolone, which could subsequently form the [a(2) - 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 [b(2) - H + Ag](+) ion, which was postulated to be an argentinated ketene.