Although plutonium is of great technological importance, the electrochemical behavior of the metal exposed to aqueous solution has not been studied. Anecdotal evidence has suggested that plutonium can be passivated. However, there have no been no electrochemical studies to date. This work is the first to examine the polarization behavior of plutonium metal. Studies using 1 mM to 1 M nitric acid solutions show that plutonium is spontaneously passive. Polarization results contradict the predicted active behavior in equilibrium potential/pH diagrams. In contrast to prior plutonium solution chemistry work using inert electrodes, a significant overpotential is required to produce hexavalent plutonium. Thus, transpassivity occurs at very noble potentials and a broad passivity range is observed in acidic solutions. Pitting is observed upon polarization in 1 mM solutions. (C) 2002 The Electrochemical Society.