Crystals of sodium hydrogen L-tartrate monohydrate (NaHC4H4O6.H2O) were grown from the aqueous solution, while potassium hydrogen L-tartrate (KHC4H4O6) crystallized as anhydrate. Their crystal structures were determined by X-ray diffraction method. The hydrogen bonds between the carboxyl and carboxylate groups in these crystals are rather long; O...O 2.533(3) and 2.531(1) angstrom for the sodium and potassium compounds, respectively. The sodium and potassium ions are coordinated by eight O atoms at the corners of distorted square antiprism, Na+...O 2.404(2)-2.826(3) angstrom and K+...O 2.718(1)-3.059(1) angstrom. The crystal and molecular structures have been compared with those of the corresponding rubidium, caesium, and ammonium salts, and discussed with the aid of semiempirical and ab initio MO methods.