Alzheimer's disease (AD) is characterized by the presence of neurofibrillary tangles and amyloid plaques, which are abnormal protein deposits. The major constituent of the plaques is the neurotoxic β‐amyloid peptide (Aβ); the genetics of familial AD support a direct role for this peptide in AD. Aβ neurotoxicity is linked to hydrogen peroxide formation. Aβ coordinates the redox active transition metals, copper and iron, to catalytically generate reactive oxygen species. The chemical mechanism underlying this process is not well defined. With the use of density functional theory calculations to delineate the chemical mechanisms that drive the catalytic production of H₂O₂ by Aβ/Cu, tyrosine10 (Y10) was identified as a pivotal residue for this reaction to proceed. The relative stability of tyrosyl radicals facilitates the electron transfers that are required to drive the reaction. Confirming the theoretical results, mutation of the tyrosine residue to alanine inhibited H₂O₂ production, Cu‐induced radicalization, dityrosine cross‐linking, and neurotoxicity.