Electric conductivity and X-ray diffraction measurements are performed on deuterated triammonium hydrogen disulphate (ND4)(3)D(SO4)(2) in high temperature phase. A new structural phase transition that corresponds to the transition at 463 K of (NH4)(3)H(SO4)(2) is found at 413 K. The space group symmetry (monoclinic C2) and the crystal structure of (ND4)(3)D(SO4)(2) are determined at 404 K in phase I. The electric conductivity due to the deuterium ion migration in phase I is discussed with respect to the results of the structure refinement.