Depth profiling measurements of light elements have been performed at the Forschungszentrum Rossendorf using an AMS facility at the 3 MV Tandetron. This facility is equipped with a simple arrangement for mechanical scanning of the sample inside a commercial sputter source and with a conventional detection system. After measurements of depth profiles of tritium in carbon samples from fusion experiments this AMS facility has been applied to depth profiling of humidity penetrated into as-implanted SiO 2 layers. For this aim, Ge + and Si + ion implanted SiO 2 layers (10 14–10 16 cm −2) were exposed to an artificial atmosphere with H 2 18O humidity. AMS allows the discrimination of isobar atomic and molecular ions, thus depth profiles of 18O − and ( 18OH) − molecular ions could be investigated. By measuring 18O 4+ or H + species of these ions, the process of humidity penetration in as-implanted SiO 2 has been studied.