This study presents the coordination modes and crystal organization of a calcium-potassium coordination polymer, poly[hexaaquabis(mu4-4-carboxybenzenesulfonato-kappa4O1:O1':O1'':O4)bis(mu3-4-carboxybenzenesulfonato-kappa2O1:O1')calcium(II)dipotassium(I)], [CaK2(C7H5O5S)4(H2O)6]n, displaying a novel two-dimensional framework. The potassium ion is seven-coordinated by four sulfonate and one carboxyl O atom located on five different acid ligands, two of which are unique, and by two symmetry-independent water O atoms. A pair of close potassium ions share two inversion-related sulfonate O-atom sites to form a dimeric K2O12 unit, which is extended into a one-dimensional array along the a-axis direction. The six-coordinate Ca2+ ion occupies a special position (1) at (0, 1/2, 1/2) and is surrounded by four sulfonate O atoms from two inversion-related pairs of unique acid monoanions and by two O atoms from aqua ligands. The compound displays a layered structure, with K2O12 and CaO6 polyhedra in the layers and aromatic linkers between the layers. The three-dimensional scaffold is open, with nano-sized channels along the c axis.