Y-6(WMo)(0.5)O-12 activated with Eu3+ ions was investigated as a red-emitting conversion phosphor for white light emitting diodes (WLEDs). The phosphors were synthesized by calcining a citrate-complexation precursor at different temperatures. The photoluminescence properties of the phosphors and the energy transfer mechanisms involved were studied as a function of structure evolution. It was found that the host lattices were crystallized in a cubic or a hexagonal phase depending on the synthesis conditions. Although all the phosphors showed intensive red emission under an excitation of near-UV or blue light due to energy transfer from the host lattices to Eu3+ ions, the photoluminescence spectra and temporal decay features were found to vary significantly with the structure and crystallinity of the host lattice. The mechanisms of the energy transfer from the host lattices to Eu3+ ions and energy quenching among Eu3+ ions were discussed on the basis of structure evolution of the host lattice. Phosphors calcined at 800 and 1300 degrees C were suggested to be promising Candidates for blue and near-UV light excited WLEDs, respectively.