[M-H](-) anions from small diphosphopeptides (phosphate groups on Ser, Thr or Tyr) show characteristic peaks corresponding to m/z 177 (H(3)P(2)O(7) (-)), 159 (HP(2)O(6) (-)) and sometimes [(M-H)(-)-H(4)P(2)O(7)](-). M/z 177 and m/z 159 are major peaks in the spectra of small peptides with 1,2, 1,3, 1,4, 1,5 and 1,6 diphosphate substitution, which means that the decomposing [M-H](-) anions must have flexible structures in order for the two phosphate groups to interact with each other. Peptides where the two phosphate groups are more than six amino acid residues apart have not been studied. Theoretical calculations indicate that m/z 177 is formed in a strongly exothermic reaction involving facile nucleophilic interaction between the two phosphate groups: m/z 159 is formed by loss of water from energised m/z 177.