The development of new efficient afterglow phosphors is currently hampered by a limited understanding of the persistent luminescence mechanism. Radioluminescence (RL) and x-ray absorption measurements on the persistent phosphor SrAl(2)O(4):Eu, Dy were combined to reveal possible valence state changes for the rare earth (co) dopants. Traps in the phosphor material are quickly filled when exposing thermally emptied SrAl(2)O(4):Eu, Dy powder to x rays. On the same time scale a partial oxidation of Eu(2+) to Eu(3+) is observed by x-ray absorption near-edge spectroscopy (XANES), while for the trivalent dysprosium the valence state remains unchanged. The impact of these observations on the recently proposed models for persistent luminescence is discussed.