The monomer and dimer structures of the amyloid fragment Aβ(1–16) sequence formed in H 2 O were investigated using electrospray ionization mass spectrometry (MS) and tandem MS (MS/MS). Aβ16 monomers and dimers were indicated by signals representing multiple proton adduct forms, [monomer+zH] n+ (=M z+ , z = charge state) and [dimer+zH] z+ (=D z+ ), in the MS spectrum. Fragment ions of monomers and dimers were observed using collision-induced dissociation MS/MS. Peptide bond dissociation was mostly observed in the D1–D7 and V11–K16 regions of the MS/MS spectra for the monomer (or dimer), regardless of the monomer (or dimer) charge state. Both covalent and non-covalent bond dissociation processes were indicated by the MS/MS results for the dimers. During the non-covalent bond dissociation process, the D 3+ dimer complex was separated into two components: the M 1+ and M 2+ subunits. During the covalent bond dissociation of the D 3+ dimer complex, the b and y fragment ions attached to the monomer, (M+b 10-15 ) z+ and (M+y 9-15 ) z+ , were thought to originate from the dissociation of the M 2+ monomer component of the (M 1+ +M 2+ ) complex. Two different D 3+ complex geometries exist; two distinguished interaction geometries resulting from interactions between the M 1+ monomer and two different regions of M 2+ (the N-terminus and C-termi- nus) are proposed. Intricate fragmentation patterns were observed in the MS/MS spectrum of the D 5+ complex. The complicated nature of the MS/MS spectrum is attributable to the coexistence of two D 5+ configurations, (M 1+ +M 4+ ) and (M 2+ +M 3+ ), in the Aβ16 solution.