The electrochemical behavior of Li/Li[NixLi(1/3-2x/3)Mn(2/3-x/3)]O-2 cells for x = 1/6, 1/4, 1/3, 5/12, and 1/2 is reported. Li[NixLi(1/3-2x/3)Mn(2/3-x/3)]O-2 is derived from Li2MnO3 or Li[Li1/3Mn2/3]O-2 by substitution of Li+ and Mn4+ by Ni2+ while maintaining all the remaining Mn atoms in the 4+ oxidation state. Conventional wisdom suggests that lithium can be removed from these materials only until both the Ni and Mn oxidation states reach 41, giving a charge capacity of 2x. We show that Li/Li[NixLi(1/3-2x/3)Mn(2/3-x/3)]O-2 cells give smooth reversible voltage profiles reaching about 4.45 V when 2x Li atoms per formula unit are removed, as expected. If the cells are charged to higher voltages, surprisingly, they exhibit a long plateau of length approximately equal to 1 - 2x in the range between 4.5 and 4.7 V. Subsequent to this plateau, the materials can reversibly cycle over 225 mAh/g (almost one Li atom per formula unit) between 2.0 and 4.8 V. In situ X-ray diffraction and differential capacity measurements are used to infer that irreversible loss of oxygen from the compounds with x < 1/2 occurs during the first charge to 4.8 V. This results in oxygen deficient layered materials with stoichiometry approximately equal to [ ] x [NixLi(1-2x)/3Mn(2-x)/3]O1.5+x at 4.8 V. These oxygen deficient materials then react reversibly with lithium.