We report a comparative Extended X-Ray Fine Structure (EXAFS) study of Er in a-Si:H prepared by Er implantation in a-Si:H and by co-sputtering undergoing the same cumulative annealing processes. It was found that the Er environment in as-implanted samples is formed by Si atoms, which are replaced by oxygen under annealing. In the co-sputtered samples, the initial low coordination oxygen environment evolves under thermal treatment to an Er2O3 -like neighborhood.