There has been a great deal of discussion on the potential role of heteroatoms in carbons intended for lithium ion intercalation. Previous observations have been mixed, ranging from beneficial to detrimental effects on intercalation capacity, irreversible losses, and charge efficiencies. The introduction or substitution of silicon into a carbon matrix prepared by chemical vapor deposition has demonstrated a positive effect. The present study concerns substitutional effects in disordered carbons synthesized from polymer precursors. Solid-state nuclear magnetic resonance, photoacoustic IR spectroscopy, X-ray fluorescence, and high-resolution transmission electron microscopy were used to follow the modified carbon synthesis. Silicon is successfully incorporated into the polymer matrix by copolymerizing the precursors with tetravinylsilane. On oxidative stabilization and pyrolysis, however, the Si-C connectivity is replaced by Si-O coordination. These silicon species appear to be discrete, and no evidence for either a crystalline or amorphous secondary SiO2 phase is seen. These silicon species have a profound effect on the electrochemical performance. Irreversible loss processes are greatly increased, and charge efficiencies are decreased when compared to nonsubstituted controls.