Solid solutions between iron and manganese cations in Li(2)FezMn((1-z))SiO(4) were successfully prepared by hydrothermal synthesis. The selected Li2Fe0.8Mn0.2SiO4 sample was electrochemically evaluated by the use of two in situ characterization techniques. Although the sample showed good reversibility with a large voltage polarization in the formation cycles, the observed electrochemical activity is only partly connected with the change in oxidation states of iron and/or manganese. Using in situ characterization techniques (Mossbauer spectroscopy and X-ray absorption near-edge structure), most of iron was reversibly oxidized to trivalent iron, whereas only a fraction of manganese was reversibly oxidized to the trivalent oxidation state. The overall change in oxidation state did not exceed more than 0.8 electron per both transition metals, even though the electrochemical experiment suggested that more than 1 electron per compound formula was exchanged. Nevertheless, the prediction that the local environment around Mn in Li2FeSiO4 (when the Mn doping is low) can be stabilized during the oxidation has been confirmed. (C) 2010 The Electrochemical Society. [DOI: 10.1149/1.3491368] All rights reserved.