An X-ray diffraction single crystal study of the hexagonal phase of Cs5H3(SO4)(4).xH(2)O, which occurs at room temperature, has been performed with the aim of determining the atomic structure and the mechanism of proton transport in this phase. M(r)=1051.79, lambda(Mo K alpha)=0.71076 Angstrom, hexagonal, P6(3)/mmc, a(h)=6.2455(8), c(h)=29.690(3) Angstrom, V=1003.0 Angstrom(3), Z=2, D-x=3.51 g.cm(-3), mu=96.1 cm(-1), F(000)=950, room temperature, R(F)=0.039, R(F)(w)=0.038 for 531 unique reflections. The results show that a Dynamically Disordered Hydrogen Bond Network (DDHBN), which is responsible for the high protonic conductivity, exists in the hexagonal phase.