The solid solution series Li[CrxLi(1/3-x/3) Mn(2/3-2x/3)]O-2 (0 < x < 1) has been synthesized using a "sol-gel'' method and studied by means of X-ray diffraction (XRD) and electrochemical techniques. XRD results are used to infer that these samples are isostructural to Li[Co]O-2, except with their transition metal layers composed of [Cr-x Li(1/3-x/3) Mn(2/3-2x/3)]. We believe the oxidation states of the elements in the transition metal layer are Li+,Cr3+, and Mn4+. Li/Li[CrxLi(1/3-x/3) Mn(2/3-2x/3)]O-2 electrochemical cells were made to study the changes in voltage profiles with composition, x. The cell with x = 1/3 delivers a reversible capacity of about 130 mAh/g between 2.0 and 4.35 V vs. Li/Li+. Between 2.0 and 4.8 V, the cell with x = 1/6 delivers a reversible capacity of about 230 mAh/g via a partial loss of oxygen during the first charge process. All the Li[Cr-x Li(1/3-x/3) Mn(2/3-2x/3)]O-2 samples have very large irreversible capacities during the first cycle.