In Alzheimer's disease, soluble A beta oligomers are believed to play important roles in the disease pathogenesis, and their levels correlate with cognitive impairment. We have previously shown that A beta oligomers can be categorized into multiple structural classes based on their reactivity with conformation dependent antibodies. In this study, we analyzed the structures of A beta 40 oligomers belonging to two of these classes: fibrillar and prefibrillar oligomers. We found that fibrillar oligomers were similar in structure to fibrils but were less stable towards denaturation while prefibrillar oligomers were found to be partially disordered. These results are consistent with previously proposed structures for both oligomer classes while providing additional structural information. (C) 2016 Elsevier Inc. All rights reserved.