The amyloid beta peptide (A beta) of Alzheimer's disease evolves hydrogen peroxide in vitro in the presence of Cu(II), external reducing agents, and molecular oxygen, without producing detectable amounts of the one-electron reduced intermediate, superoxide, O-2(-center dot). The mechanism of this process was examined by ab initio computational chemistry techniques in systems that model the binding of Cu(II) to the His13His14 fragment of A beta. The catalytic cycle begins with the reduction of the most stable Cu(II) complex to the most stable Cu(I) complex. This Cu(I) complex forms a Cu(II)-like adduct with O-3(2) that cannot dissociate in water to yield O-2(-center dot). However, it can be reduced by proton-coupled electron transfer to an adduct between HOO- and the Cu(II)-like complex, which in turn can be protonated. The protonated complex decomposes to yield H2O2 by an associative-dissociative mechanism, thus completing the cycle.