The title compound, [CuEu(2)(C(6)H(4)NO(2))(8)(H(2)O)(4)](n), displays a one-dimensional chain structure. The four-coordinate Cu(II) ion (site symmetry ) adopts a trans-CuN(2)O(2) geometry and is bridged by two carboxyl-ate groups from two isonicotinate ligands. The Eu(III) ion adopts a distorted square-anti-prismatic geometry, being coordinated by four O atoms from bridging carboxyl-ate groups of four isonicotinate ligands, two O atoms from chelating carboxyl-ate groups of one isonicotinate ligand and two O atoms from coordinated water mol-ecules; adjacent Eu(III) ions in the chain are related by inversion. The water mol-ecules interact with the ligands via O-H⋯N hydrogen bonds [O⋯O = 2.782 (3)-2.881 (3) Å], which link the chains into a three-dimensional structure.