The β-amyloid peptide (Aβ) is directly related to neurotoxicity in Alzheimer disease (AD). The two most abundant alloforms of the peptide co-exist under normal physiological conditions in the brain in an Aβ42:Aβ40 ratio of ∼1:9. This ratio is often shifted to a higher percentage of Aβ42 in brains of patients with familial AD and this has recently been shown to lead to increased synaptotoxicity. The molecular basis for this phenomenon is unclear. Although the aggregation characteristics of Aβ40 and Aβ42 individually are well established, little is known about the properties of mixtures. We have explored the biophysical and structural properties of physiologically relevant Aβ42: Aβ40 ratios by several techniques. We show that Aβ40 and Aβ42 directly interact as well as modify the behavior of the other. The structures of monomeric and fibrillar assemblies formed from Aβ40 and Aβ42 mixtures do not differ from those formed from either of these peptides alone. Instead, the co-assembly of Aβ40 and Aβ42 influences the aggregation kinetics by altering the pattern of oligomer formation as evidenced by a unique combination of solution nuclear magnetic resonance spectroscopy, high molecular weight mass spectrometry, and cross-seeding experiments. We relate these observations to the observed enhanced toxicity of relevant ratios of Aβ42:Aβ40 in synaptotoxicity assays and in AD patients.