Mixed rare-earth organophosphates have been investigated as potential corrosion inhibitors for AA2024-T3, and previously have shown synergistic inhibition behavior; however, the mechanism was not identified. In this paper, a key factor contributing to corrosion inhibition of AA2024-T3 with mischmetal diphenyl phosphate [Mm(dpp)(3)] is the unique stability of Pr(dpp)(3) compared to other key rare earths in mischmetal. Although increasing pH causes precipitation of other components, the Pr compound is stable at higher pH. Electrochemically, a synergy is evident when Ce(dpp)(3) and Pr(dpp)(3) are combined. Raman mapping indicates the Pr(dpp)(3) inhibitor leads to a more uniform coverage of the alloy. (c) 2007 The Electrochemical Society.