Disulfide bonds in gaseous multiply-protonated proteins are preferentially cleaved in the mass spectrometer by low-energy electrons, in sharp contrast to excitation of the ions by photons or low-energy collisions. For S-S cyclized proteins, capture of one electron can break both an S-S bond and a backbone bond in the same ring, or even both disulfide bonds holding two peptide chains together (e.g., insulin), enhancing the sequence information obtainable by tandem mass spectrometry on proteins ill trace amounts. Electron capture at uncharged S-S is unlikely; cleavage appears to be due to the high S-S affinity for H* atoms, consistent with a similar favorability found for tryptophan residues. RRKM calculations indicate that H* capture dissociation of backbone bonds in multiply-charged proteins represents nonergodic behavior, as proposed for the original direct mechanism of electron capture dissociation.