The electrochemical mechanism leading to the pseudocapacitance of MnFe2O4 in aqueous electrolyte solutions has been investigated by, in addition to electrochemical characterization, synchrotron X- ray absorption near-edge spectroscopy (XANES) and X- ray diffraction (XRD) analyses using KCl solution as the model electrolyte. The ferrite crystallites, prepared by a solution method, contain Mn and Fe ions randomly distributed over the tetrahedral and octahedral sites of the spinel structure and exhibit an average capacitance exceeding 100 F/g-ferrite. Based on the data from the in situ XANES and XRD measurements and open-circuit- potential dependence on solution pH, it is concluded that the pseudocapacitance of MnFe2O4 involves charge transfer at both the Mn- and Fe- ion sites, balanced by insertion/ extraction of proton into/ from the lattice. In addition, compared with MnO2 . nH(2)O electrode, the ferrite exhibits a far reduced, by similar to 90%, lattice expansion upon cycling. (c) 2006 The Electrochemical Society.