Single crystalline α-Al2O3 was implanted with several fluences of Co ions. Detailed angular scans performed after implantation indicate incorporation of the Co ions into substitutional Al sites for fluences up to 1×1015cm−2. For higher fluences a large amount of defects are produced and new compounds start to form in the implanted region. At a fluence of 5×1017cm−2 the samples displayed both metallic conduction and ferromagnetic behaviour. Annealing in an oxidising atmosphere promote the diffusion of Co into the bulk leaving a Co-rich thin layer at the surface and leading to the formation of a Co–Al–O compound that extends to a depth of 250 nm. After this annealing the magnetic behaviour disappears and the samples become insulating. Annealing in a reducing atmosphere produces a Co-rich layer in the topmost 80 nm of the surface without loss of the ferromagnetic behaviour and the metallic conductivity.