It was found that the lower photoluminescence of hydrothermal YBO3: Tb under 147 nm excitation is mainly due to the presence of Tb4+. The electron paramagnetic resonance spectra confirmed the existence of Tb4+ in hydrothermal YBO3: Tb, and the Tb4+ was difficult to eliminate completely by calcination in air. The emission spectra showed the Tb4+ induced redundant D-5(3)- F-7(J) emission of Tb3+. The results of an infrared study revealed hydrothermal YBO3: Tb contains some BO33- and Tb4+ only exists in hydrothermal phosphors which contain planar triangular groups. The Tb4+ was eliminated completely by calcination in a reducing atmosphere, and it greatly enhanced photoluminescence of hydrothermal YBO3: Tb in vacuum ultraviolet (VUV) region. (c) 2006 The Electrochemical Society.