In the centrosymmetric polymeric title compound, {[CoEr(2)(C(14)H(8)N(2)O(5))(4)(H(2)O)(4)]·4H(2)O}(n), the Er(III) cation has a coordination number of eight and is surrounded by seven carboxyl-ate O atoms from four 5-(pyridine-4-carboxamido)-isophthalate (L) ligands and one water mol-ecule, forming a distorted square-anti-prismatic arrangement. The Co(II) cation is located on an inversion center and is coordinated by two pyridine N atoms, two carboxyl-ate O atoms and two water mol-ecules in a distorted octa-hedral geometry. The asymmetric unit contains two anionic L ligands. One bridges two Er(III) cations and one Co(II) cation through two carboxyl-ate groups and one pyridine N atom, while the other bridges two Er(III) cations and one Co(II) cation through two carboxyl-ate groups. Extensive O-H⋯O, O-H⋯N and N-H⋯O hydrogen-bonding inter-actions are present in the crystal, involving all uncoordinated water mol-ecules and the uncoordinated pyridine N atom of one of the ligands bonded to an adjacent coordinated water mol-ecule. The title compound is isotypic with the gadolinium analogue.