Starting from pitches with oxygen and sulfur contents varying from 0.13 to 2.7% by weight, we prepared a series of pregraphitic carbons by pyrolysis at 1000 degrees C. The presence of O and S has no apparent physical or structural effect on the final carbon, However, a direct link was found between oxygen and sulfur contents and the irreversible capacity of the electrochemical reaction of Li with these carbons. Apparently Li reacts irreversibly with sulfur and oxygen to form Li-O and Li-S bonds as are found in Li2O and Li2S. According to this model, the irreversible capacity should be directly proportional to the chalcogen content, and this is approximately observed at high chalcogen content. For lower concentrations, the formation of the solid electrolyte interphase (SEI) is the dominant cause of Li loss,