Bi3+ and/or Eu3+ co-doped/undoped (Y-0.65,Gd-0.35)BO3 were synthesized and their vacuum ultraviolet (VUV)/ultraviolet excitation and emission spectra were investigated using synchrotron radiation. We found that the dominant energy transfer from Bi3+ to Eu3+ in (Y-0.65,Gd-0.35)BO3:Bi-0.01(3+),Eu-0.04(3+) is Bi3+-> Gd3+...Gd3+-> Eu3+. The absorbed energy first relaxes to the P-1(1), P-3(2) or P-3(1) of Bi3+, and then transfers to the (6)G(J), D-6(J), I-6(J), or P-6(J) of Gd3+. The excitation state of Gd3+ migrates over the Gd sublattice until trapped by an Eu3+, where it relaxes to D-5(0), and finally radiatively returns to the ground state. (c) 2007 The Electrochemical Society.