Si/C composites were synthesized by the co-carbonization of 3,5-xylenol-derived novolac resin with various polysilane polymers. A significant increase in the Li+ reversible capacity due to the inclusion of Si in the C matrix was observed. However, the Li+ irreversible capacities of the materials were also fairly large. The irreversible Li+ capacities were fand to be explainable by two factors: the consumption of Li+ due to the formation of solid electrolyte interface layers within the materials, the amount of which depends on the pore structure of the material, and the trapping of Li+ by the O included in the material. It was also found that the Si in the materials synthesized using polydiphenylsilane as the Si source could store up to an average of 2.4 Li+ per atom. These results indicate that by selecting a proper C source for the co-carbonization with polysilane polymers, an anode material for Li+ batteries with a large reversible capacity and small irreversible capacity can be obtained.