Two types of LiNi0.90Mn0.10O2 compounds have been synthesized by a coprecipitation method in the presence of either 5 or 50% lithium excess. The structural characterization by X-ray and neutron diffraction, associated with Rietveld analysis, has shown the presence of 5 or 3% extra nickel ions in the interslab space, respectively. Cycling tests have shown a decrease of the electrochemical properties for the manganese-substituted materials, with a reversible capacity of about 110 mAh/g in the 3-4.15 V range at the C/20 rate. (C) 2003 The Electrochemical Society.