In the crystal structure of the title hydrated salt, 2C(4)H(12)N(+)·C(2)O(4) (2-)·H(2)O, the two independent cations, the anion and the water mol-ecule all lie on special positions of m site symmetry. In both cations, the mirror plane passes through the nitrogen atom and two methyl groups; in the anion, the mirror plane passes through two carbon and two oxygen atoms. The anions and water mol-ecules inter-act by O-H⋯O hydrogen bonding, forming a chain running along the b axis.