Krypton ions were injected into Pyrex glass at temperatures of 145, 210 and 270°C. The glass was then bombarded with argon ions at various temperatures below the primary ion injection temperature and the rate of release of the primary species observed. The variation of the bombardment-induced gas release rate with temperature shows a rapid increase as the secondary bombardment temperature approaches the primary injection temperature. These measurements, and observations of the preferential removal of gas trapped with low activation energies, suggest the operation of a thermal spike process (i.e. an effective violent local heating of the glass near the point of ion impact) which causes gas release.