Reaction in an aqueous gel medium between a lanthanide ion and the sodium salt of the naphthalene-tetra-carboxylic acid can lead to coordination polymers. We report here the synthesis, the crystal structure and the calculated porosity of the first two new coordination polymers, namely Gd[(C10H4)(COO)3(COOH)](H2O)4·9H2O and Er4[(C10H4)(COO)4]3(H2O)10·12H2O. The first compound crystallizes in the triclinic system, space group P¯1 (no. 2) with a = 6.4456(1) A° , b = 12.0131(3) A° , c = 16.4256(4) A° , α = 107.8706(9)◦, β = 97.0735(9)◦, γ = 93.4771(15)◦ and Z=2. The crystal structure can be described as wavy planes strongly connected to each other by hydrogen bonds. This pseudo 3D crystal structure presents very large rectangular channels filled by crystallization water molecules. The second compound crystallizes in the monoclinic system, space group P21/c (no. 14) with a = 9.8442(1) A° , b = 29.2288(2) A° , c = 10.8795(1) A° , β = 92.1767(3)◦ and Z = 4. Its crystal structure is 3D and presents some large channels with rectangular sections. These channels are full of crystallization water molecules. In both cases, the crystallization water molecules have been formally removed and the porosity has been calculated using Connolly's algorithm. Both present a rather high potential porosity.