The reaction of thin Co films with Si and SiO2during ion-beam mixing with As and Ge was studied. Fluences and energies used were varied between 2-1014and 5-1015cm−2, and 15 and 200 keV, respectively. Concentration profiles of As, Ge, and Co were measured by SIMS; cross-sectional TEM-analysis was applied in order to investigate the metal-silicon interface. By comparing theoretical and experimental profiles, it was possible to separate ballistic effects from thermal processes in the Co-Si system. All theoretical profiles were calculated using the dynamic Monte-Carlo simulation program T-DYN. In case of Co-SiO2interaction, it could be shown that the Co distribution in the oxide was only caused by recoil implantation and not by diffusion processes or reactions.