The formation and release of hydro‐ and deuterocarbons from pyrolytic graphite bombarded with 6‐keV H+3ions and subsequently with 6‐keV D+3ions as well as vice versa have been investigated using a quadrupole mass spectrometer (QMS) at a target temperature of 500 °C. For both cases the signal intensities of masses 17 (CHD2,CH3D) and 19 (CHD3) first increased and then decreased with increasing the fluence of subsequent bombardment. A similar tendency was also observed in those of masses 29 (C2H3D, C2HD2) and 31 (C2HD3). These results indicate that the mixing of hydro‐ and deuterocarbons, forming CHmDnand C2HmDn, occurs during the subsequent bombardment. A simple calculation of QMS signal intensities has been performed, assuming that the fully saturated hydro‐ and deuterocarbons (CH4, C2H2, and C2H4types) are formed and then released, and that their radicals are only produced through cracking in the ion source of QMS. The calculated results qualitatively agreed with the measured ones. They suggest that the mixing of hydro‐ and deuterocarbons becomes maximum when the product of the local H and D concentrations has the maximum value, and that the mixing mainly takes place around the depth at which the H and D profiles mostly overlap.