Oxidative stress is linked to the etiology of Alzheimer’s disease (AD), the most common cause of dementia in the elderly. Redox active metal ions such as copper catalyze the production of Reactive Oxygen Species (ROS) when bound to the amyloid-b (Ab) peptide encountered in AD. We propose that this reaction proceeds through a low-populated Cu–Ab state, denoted the “catalytic in-between state” (CIBS), which is in equilibrium with the resting state (RS) of both Cu(I)–Ab and Cu(II)–Ab. The nature of this CIBS is investigated in the present work. We report the use of complementary spectroscopic methods (X-ray absorption spectroscopy, EPR and NMR) to characterize the binding of Cu to a wide series of modified peptides in the RS. ROS production by the resulting Cu–peptide complexes was evaluated using fluorescence and UV-vis based methods and led to the identification of the amino acid residues involved in the Cu–Ab CIBS species. In addition, a possible mechanism by which the ROS are produced is also proposed. These two main results are expected to affect the current vision of the ROS production mechanism by Cu–Ab but also in other diseases involving myloidogenic peptides with weakly structured copper binding sites.