The electrochemical behavior of the SiO (silicon monoxide) anode during its reaction with Li has been investigated using solid-state nuclear magnetic resonance (NMR) and high resolution transmission electron microscopy (HRTEM). It was mainly focused on identifying the intermediate and final phases formed with their microstructures of the electrochemically lithiated SiO at various voltage steps during the first cycle. A model for the reaction of SiO with lithium was suggested, which was consistent with the discharge capacity of about 2400mAhg−1.