Activated bixbyite oxides (e.g. Eu:Lu 2 O 3 ) are being considered as radiation detectors. In an attempt to improve their optical efficiency and decrease afterglow, these compounds have been doped with aliovalent cations. Here, atomistic scale computer simulation has been used to predict the defect processes associated with the solution of extrinsic divalent and tetravalent ions. These calculations provide a mechanistic framework through which it is possible to identify how specific doping schemes modify the populations of defects that could influence scintillator performance. A change in solution site preference is predicted for both divalent and tetravalent solutions as a function of dopant and host lattice cation radii.