The rhombohedral-rhombohedral phase transition of (NH4)(3)H(SeO4)(2) has been investigated by means of precise temperature dependent X-ray powder diffraction. Lattice parameters and thermal expansion parameters have been determined within the temperature range from 380 to 311 K in steps of 3 K. Their temperature dependent behaviour suggest that the phase transition at 337 K is continuous. At about 354 K a kink in the course of the c-lattice parameter has been detected which might be related to a new phase transition. The structure is discussed with respect to the observations.