The CoII cation in poly[[aqua(μ-benzene-1,2-dicarboxylato-κ3
O
1,O
2:O
1)(μ-4,4′-bipyridine-κ2
N:N′)cobalt(II)] trihydrate], {[Co(C8H4O4)(C10H8N2)(H2O)]·3H2O}
n
, is octahedrally coordinated by two N atoms of two 4,4′-bipyridine ligands, three O atoms from phthalate anions and a fourth O atom from a coordinated water molecule. The packing consists of planes of coordination polymers linked by hydrogen bonds mediated by three solvent water molecules; the linkage is achieved by the water molecules forming intricate oligomeric clusters which also involve the O atoms of the phthalate ligands.