The title compound, K(+).[(C(8)H(5)O(4))(2)H](-).2H(2)O or K(+).C(16)H(11)O(8)(-).2H(2)O, was prepared by slow evaporation of an aqueous solution of potassium hydrogen phthalate. The molecular complex consists of a potassium cation coordinated to a proton-bound hydrogen phthalate dimer and two water molecules. The potassium cation resides on a twofold axis in a distorted square-antiprism coordination geometry. The compound is isomorphous with the ammonium analogue, previously misidentified. As potassium hydrogen phthalate is frequently used in the manufacture of buffers, organic carbon standards, acidimetric standards and various other products, the crystallization of a compound with a different stoichiometery from a solution containing the acidimetric standard has important practical implications.