CR-39, a polymer widely used for nuclear track detectors, was bombarded with 500 keV protons and Ar ions, and with 2.8 MeV He ions. A mass analysis of the gas ejected during bombardment yielded CO2, CO, and H2. In the case of He ions, the amount of hydrogen was monitored during the bombardment by using elastic recoil analysis. The hydrogen spectrum was unchanged in spite of the gas loss, and it was concluded that the bombarded volume was being continuously replaced by new material, with very little carbon buildup. Since the bombardment leaves a clearly visible depression on the surface, the volume of gas ejected could be measured using a profile measuring apparatus. From this it was calculated that each 2.8 MeV He ion produces 23,000 ejected atoms. Since the range of these ions is 12.8μm, an ejection rate of one atom every 0.5 nm is determined, essentially half the atoms that the ion encounters. For 500 keV protons and argon ions, there are one atom ejected every 0.9nm and one atom every 0.16nm, respectively. We therefore propose that this gas ejection will contribute in an important way to the formation of etchable tracks.