MgO implanted at room temperature with 150keV In+ions and doses ranging from 1015to 1017ions cm−2was studied by optical absorption and transmission electron microscopy (TEM). Creation of defects in the anionic sublattice (F-, F+-, F2-centers) and in the cationic sublattice (V−-centers) are observed. Subsequent annealings of the implanted crystals have shown different behaviours depending on the implanted dose. For medium dose (2 × 1016ions cm−2), the formation of In3+species seems to be the preponderant phenomenon. At higher dose (8 × 1016ions cm−2), metallic precipitates are formed between 400 and 700°C. The identification of these precipitates has been achieved using TEM: they are formed of a metallic alloy Mg3In with a hexagonal structure and their orientation relationship with respect to the MgO matrix is: (0001)Mg3In//(111)Mgo and [1120]Mg3In// [l10]MgO.