Ln(2)B(6)O(10)(OH)(4)center dot H2O (Ln = Pr, Nd, Sm-Gd, Dy, Ho, and Y), a new series of hydrated rare earth borates, have been synthesized under hydrothermal conditions. A single crystal of Nd analogue was used for the structure determination by X-ray diffraction. It crystallizes in the monoclinic space group C2/c with lattice constants a = 21.756(4), b = 4.3671(9), c = 12.192(2) angstrom, and beta = 108.29(3)degrees. The other compounds are isostructural to Nd2B6O10(OH)(4)center dot H2O. The fundamental building block (FBB) of the polyborate anion in this structure is a three-membered ring [B3O6(OH)(2)](5-). The FBBs are connected by sharing oxygen atoms forming an infinite [B3O5(OH)(2)](3-) chain, and the chains are linked by hydrogen bonds, establishing a two-dimensional (2-D) [B-610(OH)(4)center dot H2O](6-) layer. The 2-D borate layers are thus interconnected by Ln(3+) ions to form the complex three-dimensional structure. Ln(2)B(6)O(10)(OH)(4)center dot H2O dehydrates stepwise, giving rise to two new intermediate compounds Ln(2)B(6)O(10)(OH)(4) and Ln(2)B(6)O(11) (OH)(2). The investigation on the luminescent properties of Gd2-2xEu2xB6O10(OH)(4)center dot H2O (x = 0.01-1.00) shows a high efficiency of Eu3+ f-f transitions and the existence of the energy transfer process from Gd3+ to Eu3+. Eu2B6O10(OH)(4)center dot H2O and its two dehydrated products, Eu2B6O10(OH)(4) and Eu2B6O11(OH)(2), present the strongest emission peak at 620 nm (D-5(0) -> F-7(2) transition), which may be potential red phosphors.