Wide-bandgap materials doped with rare-earth ions are currently of great interest as new vacuum ultraviolet (VUV) phosphors for lighting and displays. Ibis paper reports the development of a highly sensitive green phosphor, CaSO4:Tb,Na, which exhibits a quantum efficiency higher than 100% by exploiting the energy-transfer mechanism from anion excitons to the activator ions, Tb3+. The VUV excitation spectra of CaSO4:Tb3+ with Na+ as a charge compensator show two prominent excitation bands at 147 and 216 nm. The former band is attributed to the charge-transfer excitations within SO42- complexes while the latter was assigned to the 48 -> 4f(7)5d transitions on Tb3+. The energy-transfer mechanism from anion excitons to Tb3+ strongly raises the possibility of two-photon emission via a second-order down-conversion under the VUV excitation, which is basically a new approach in the goal of achieving a quantum-splitting phosphor.