We investigated the electrochemical behaviors of nonstoichiometric silicon suboxides (SiO x , for x = 0.17–1.34) as the anode material for lithium rechargeable batteries. The amorphous SiO x (a-SiO x ) films were synthesized by reactive evaporation of Si with oxygen gas. The initial charge (lithiation) and discharge (delithiation) capacities were strongly related to the value of x in SiO x . We proposed an electrochemical reaction model of SiO x with lithium (Li) to describe the relationships quantitatively by assuming SiO x was composed of a reversible Si and an irreversible SiO2 component. Lithiated products were identified by XPS as a fully lithiated Si, Li2O, Li silicates (Li4SiO4, Li2SiO3), and SiO2. After discharge, a portion of the Li in Li–Si was found to remain undischargeable as Li z Si (z; the function of x). This was an additional factor of the irreversible capacity. Peak positions assigned to Li z Si in XPS Si2p spectra suggest that the value of z depends on the value of x. The a-SiO x (for x = 1.02, 1.34) films demonstrated excellent cyclability at a range of 0.005–1.5 V vs. Li/Li+. Absence of peaks at 0.45 V in differential capacity vs. voltage (dQ/dV) profiles indicates the suppression of crystallization of lithiated a-Si, even under deep cycle conditions.