The structure of (NE4)(4)H-2(SeO4)(3) and of its deuterated analogue was re-determined at room temperature and at similar to 140 K. In the examined temperature range, the same centrosymmetric space group P (1) over bar was ascribed to both crystals. A continuous temperature variation of lattice parameters was still apparent there. Splitting of the Raman band at 867 cm(-1). arising from nu (1) vibrations of two-proton donor selenate tetrahedra in (ND4)(4)D-2(SeO4)(3), was observed below 215 K. The low-temperature dependence of dielectric permittivity of (NH4)(4)H-2(SeO4)(3) exhibits a broad maximum which we relate to a glass-like transition and not to a ferroelectric-paraelectric one.