Photo-, radio-, thermo-luminescence properties and afterglow decay kinetics are reported for Gd2O2S:Tb3+ X-ray phosphor samples co-doped by Ca2+, Ta5+, Sn4+, and Ru4+ ions. A strong influence of co-doping on the optical properties was found, which can be explained by the influence of the excess or lack of charge in the lattice due to aliovalent ions on the concentration of point defects. Possible microscopic processes are proposed. (C) 2003 The Electrochemical Society.