This work reports an interesting result on the photoluminescence OfYP04. 21420 doped with low concentration of Tb which was synthesized at low temperature by aqueous route. In the as-synthesized Y0.098PO4:Tb-0.002 . 2H(2)O, in which the hydration water is coordinated to the rare earth ions, the emissions of D-5(4) of Tb3+ are much more intense than the emissions of D-5(3). After dehydration by annealing, an opposite result is achieved, i.e., the emissions of D-5(4) of Tb3+ are much lower than the emissions of D-5(3). The Fourier-transform infrared (FTIR) and fluorescent decay spectra demonstrate that the coordinated water molecule in the hydrated sample contributes to the relaxation of D-5(3) to D-5(4), thus increasing the population of the excited state D-5(4) of Tb3+ in the as-synthesized sample. (c) 2007 Elsevier B.V. All rights reserved.