[M[BOND]H] parent anions of underivatised peptides containing an intramolecular disulfide bridge undergo characteristic loss of the elements of H2S2, a process diagnostic of the presence of the disulfide moeity. This facile process is initiated from a side-chain enolate anion. Theoretical calculations (at the HF/6-31G(d)//AM1 level of theory) indicate that the process is exothermic with a small barrier. When the disulfide link involves a C-terminal Cys, the negative ion spectrum shows an [(M[BOND]H)[BOND](H2S2+CO2)] fragment anion which is usually the main peak of the spectrum. This process is also directed by an enolate anion: theoretical calculations suggest a stepwise sequence with loss of CO2 preceding loss of H2S2. Both [(M[BOND]H)[BOND]H2S2] and [(M[BOND]H)[BOND](H2S2+CO2)] anions undergo backbone cleavage allowing identification of the amino acid sequence of the peptide. Copyright © 2005 John Wiley & Sons, Ltd.