The title compound, {[Er2(C4H2O4)2(C2O4)(H2O)4]·4H2O} n , was synthesized by the reaction of erbium nitrate hexa-hydrate with fumaric acid and oxalic acid under hydro-thermal conditions. The Er(3+) cation (site symmetry ..2) is eight-coordinated by six O atoms from four fumarate anions (site symmetry ..2) and one bidentate oxalate anion (site symmetry 222), and by two water mol-ecules. The complex exhibits a three-dimensional structure consisting of oxalate pillared Er-fumarate layers with channels occupied by coordinating and lattice water mol-ecules. The three-dimensional structure features by Owater-H⋯O hydrogen bonds involving both the coordinating and lattice water mol-ecules.