Thin films of Si1-xOx were produced using combinatorial sputtering methods. X-ray diffraction studies of these films show that they are amorphous or nanostructured. The effect of oxygen content on the electrochemical performance of these materials was studied. The reversible specific capacity (between 0.005 and 0.9 V) agrees with the assumption that Si1-xOx is made up of amorphous silicon, which can react to form Li3.75Si, and amorphous SiO2, which reacts to form Li4SiO4. The irreversible capacity increases with oxygen content when measured to a potential limit of 0.9 V vs Li/Li+ but further Li can be extracted from sites presumably near oxygen up to a potential of 2.0 V vs Li/Li+. However, such potentials for the negative electrode would not be reached in a full Li-ion cell. This work shows that the oxygen content in Si1-xOx should be optimized to produce materials with a reasonable active/inactive ratio in order to have electrodes with the desired specific capacity, an appropriate irreversible capacity and an appropriate amount of inactive phase to buffer volume expansion. (C) 2013 The Electrochemical Society. All rights reserved.