Among the silicon (Si)-based anode materials, Si/SiOx is likely to be the best candidate for maintaining structural stability while taking advantage of the high capacity of Si as well as the compatible functionality between Si and SiOx. The oxygen content of SiOx is an especially significant factor for obtaining better performance, because it directly affects electrochemical properties such as reversible capacity, structural stability, and electrical conductivity. Herein, we have synthesized SiOx materials with different oxygen contents via magnesiothermic reduction of silica. The theoretical capacities of a series of Si/SiOx samples were estimated by using X-ray photoelectron spectroscopy, and then those results were compared with empirical results. This work paves the way to predicting the electrochemical performances of lithium-ion battery anodes based on oxygen content in the nonstoichiometric Si/SiOx structure.