The first open-framework lanthanide carboxyphosphonate has been obtained under hydrothermal conditions. The three-dimensional structure of MIL-84(Pr) (MIL = Material Institut Lavoisier) or Pr-4(H2O)(7)[O2C-C5H10N-CH2-PO3](4)-(H2O)(5) has been solved from X-ray diffraction single-crystal data (a = 23.481(1) Angstrom, b = 10.159(1) Angstrom, c = 23.006(1) Angstrom, beta = 105.63(1)degrees, V = 5284.6(6) Angstrom(3), space group Cc (No. 9)). Its framework is built up from chains of edged-sharing eight or nine-coordinated monocapped square antiprism polyhedra and carboxyphosphonate anions, creating a three-dimensional structure with small pores filled with water molecules. The thermal behavior of MIL-84(Pr) has been investigated using TGA and X-ray thermodiffractometry and indicates that MIL-84(Pr) is stable up to 523 K with a reversible hydration-dehydration process. The optical study of its yttrium analogue doped at 3.4% with europium (MIL-84(Y,Eu)) reveals a significant red-orange emission under UV radiation.