Magnesium-substituted lithium manganese orthosilicate (Li(2)MnSiO(4)) cathode materials with a nominal composition of Li(2)Mg(x)Mn(1-x)SiO(4), for x = 0.4 and 0.5 are synthesized by a solid-state route, at 700 degrees C in argon. The samples are characterized using powder X-ray and neutron diffraction, scanning electron microscopy, and galvanostatic cell-cycling. Rietveld analyses of the powder X-ray and neutron diffraction data show the formation of a monoclinic P2(1)/n structure related to gamma lithium phosphate with no significant impurity peaks. This structure of the Mg-substituted samples is in contrast to the unsubstituted Li(2)MnSiO(4) compound that has a Pmn2(1) structure when synthesized under the same conditions. Unit-cell volumes of the Mg-substituted materials are intermediate between those of the P2(1)/n structure of Li(2)MnSiO(4) and the isostructural low-temperature form of Li(2)MgSiO(4), indicating the formation of a solid solution. The Mg-substituted materials feature mixed Mg/Mn cation sites, although no evidence of Li/Mn, Li/Mg or Li/Mg/Mn mixed sites are found. The Li(2)Mg(x)Mn(1-x)SiO(4) cathodes show improved electrochemical performance over that reported for the unsubstituted Li(2)MnSiO(4) P2(1)/n phase. The Li(2)Mg(x)Mn(1-x)SiO(4) cathode performance remains limited by its poor electronic properties and the large particle size of the solid-state synthesized products. Optimization of the synthesis conditions is likely to lead to enhanced electrochemical performance. (C) 2011 Elsevier B.V. All rights reserved.