The irreversible capacity of carbon electrodes for lithium is a critical parameter which must be minimized in practical Li ion cells. This paper reports studies of the origin of the irreversible capacity in high-capacity carbons derived from sugar, and for comparison, in graphitic carbons made from mesocarbon microbeads. Tablet electrodes (similar to 0.4 mm thick) of sugar carbon or of mesocarbon microbeads (mixed with 10% pitch) were prepared by pyrolysis of the precursors at 1050 degrees C under a vacuum of about 10 mTorr. Tablets exposed to different gases were used in carbon/Li coin cells, in order to study the effect of different gas exposures on irreversible capacities, C-irr, for ii insertion. Carbon electrodes exposed only to argon or nitrogen demonstrated a dramatic reduction in irreversible capacity : C-irr approximate to 50 and 10 mAh/g for sugar carbon and mesocarbon microbead tablet cells, respectively, compared to C-irr approximate to 180 and 30 mAh/g for the corresponding conventional doctor-blade spread electrode cells where the electrodes had been exposed to laboratory air for several days. Tablet electrodes were also exposed to CO2, O-2, water, steam, and air, respectively, for different periods of time. Tablets exposed to each of these reactive gases showed a dramatic increase in irreversible capacity. The irreversible capacity for hard carbons results from two sources : (i) electrolyte decomposition on nominally clean carbon surfaces and (ii) reactions with surface groups which form on carbons exposed to reactive gases. The amount of irreversible capacity from the latter source depends on the gas exposure time and involves reactions with species such as hydroxyl, carboxyl functional groups, or adsorbed water.