The reaction of CaCO3 with isophthalic acid in water yields nonaaquapenta-mu-isophthalato-pentacalcium octahydrate, [[Ca(5)(C(8)H(4)O(4))(5)(H(2)O)(9)].8H(2)O](n), a complex polymeric one-dimensional column structure bearing metal-carboxylate bonds and Ca-bound terminal and bridging water molecules, in addition to hydrogen-bonded water molecules of crystallization. The asymmetric unit comprises half of the formula unit, with one Ca(2+) ion located on a twofold axis, and contains 16 unique strong O-H.O hydrogen bonds, some of which link the columns together.