Collision-induced dissociation (CID) with ESI-MS was used to obtain structural information on early-stage aggregation of hIAPP (human islet amyloid polypeptide) monomers and oligomers. MS analysis showed that the hIAPP monomers and oligomers were multiply charged. MS/MS analysis indicated that the hIAPP monomers adopt different structures depending on the parent ion charge state. The fragmentation patterns indicated structural similarity of M2+ & M3+ and M4+ & M5+ (M = monomer). MS/MS analysis of the dimers showed that D5+ (D = dimer) comprised M2+ and M3+ subunits, and the peptide bond dissociated in the 15-37 residue region of the monomer subunit.