In this paper, a novel three-dimensional (3D) porous lanthanide-organic framework, Eu-2(mu(4)-pmdc)(2)(OH)(2)center dot 3H(2)O (1), which is stable up to 400 degrees C, has been hydrothermally synthesized and characterized. It shows intriguing single-crystal-to-single-crystal transformation and reversible dehydration/rehydration phenomenon upon removal and rebinding of the lattice water molecules, which is supported by single-crystal X-ray diffraction, powder X-ray diffraction, and photoluminescence data.