The irreversible capacity of disordered carbons, used as anodes of lithium-ion batteries, is known to result from their generally large surface area (leading to large volume of passivating layer) and from the presence of surface functional groups and of heteroatoms left from the precursor, which can react irreversibly with lithium. In this paper, we show that the irreversible capacity of disordered carbons can result from an additional mechanism: lithium trapping in the internal porosity. A series of carbon samples with low specific surface area and low heteroatom content has been investigated by X-ray diffraction, helium pycnometry, complex impedance spectroscopy and galvanostatic cycling. A clear correlation has been found between the irreversible capacity and the internal pore volume.