Though it is generally accepted that fast proton transport in superionic hydrogen sulphate and selenate crystals proceeds via Grotthuss mechanism and conditions of intra- and intermolecular proton transport were discussed considerably, there was no structural data support available. We report here results of X-ray studies of (NH4)(3)H(SeO4)(2) single crystals in superionic phases II and I. Mean thermal amplitudes of the O(2) oxygens from neighbouring SeO4 in the (001) plane were calculated on the basis of riding model analysis of experimentally obtained anisotropic temperature factors. It is shown that thermal excitations of SeO4 tetrahedra in the superionic phases result in a variety of O(2)-O(2) separation length conditioning both the near barrierless proton transfer and breaking of hydrogen bonds in the (001) plane.