The occurrence of O 2 molecular loss from the bulk of SiO 2 single layers and SiO 2 /Si multilayers as a result of 50 MeV Cu 9 + irradiation has been investigated. This process did not take place with a significant rate, if it occurs at all. Instead both Si and O are removed from the SiO 2 surface region, releasing molecular O 2 . If an elemental Si layer is on top in a multilayer, removal of Si and O with an appreciable rate is not observed. The irradiation creates bubbles in the SiO 2 /Si multilayers, which contain O 2 . The distinct SiO 2 sublayers remain chemically intact. The bubbles deteriorate the depth resolution in elastic recoil detection.