Abstract: Dehydrated ions of 16 protonated peptides that contained α- or β-Aspartic isomers in 2 and 6 position respectively were generated by ESI-MS and low-energy collision-induced dissociation (CID). The relative ion abundance ratios of b_n-H₂O/b_n increased when α-Asps were converted into β-Asp in the peptides studied with the exception of b₆-H₂O/b₆ ratios that decreased for the peptides 8, 10, 12, 14, 16; the ratios of M＋H－H₂O^＋/M＋H^＋ also decreased. It shows that the quantitative comparison of the dehydrated and corresponding non-dehydrated ions can be used for distinguishing α- or β-Asp isomers in the peptides.