This work reports the Li+ uptake/extraction mechanism in silicon monoxide ( SiO) as the negative electrode in lithium secondary batteries. A combined study of solid-state Si-29- and Li-7-nuclear magnetic resonance (NMR) , electrochemical dilatometry, and charge-discharge cycling consistently demonstrates that the SiO2 domain in SiO irreversibly reacts with Li+ to produce lithium silicates and Li2O in the first discharging period, whereas the elemental Si domain reversibly reacts, delivering the same chargedischarge characteristics to those of conventional amorphous Si electrodes. The volume expansion accompanied by the irreversible reaction is less significant than that caused by the lithiation of Si domain. The postmortem analysis made on cycled electrodes reveals a phase segregation between the lithium silicates/ Li2O and lithiated Si phase. It is likely that the lithium silicates/Li2O phase plays a buffering role against the volume change of Si matrix, but the crack formation at the phase boundaries and eventual pulverization are still a problem to be solved. (c) 2007 The Electrochemical Society.