An X-ray diffraction single crystal study of the hexagonal high-temperature phase of Cs5H3(SeO4)4.H2O has been performed with the aim of determining the atomic structure and the mechanism of proton transport in this phase. M(r)= 1257.39, lambda(MoKalpha)=0.71076 angstrom, hexagonal, P6(3)/mmc, a(h)=6.400(3), c(h)=30.03(1) angstrom. V=1065.2 angstrom3, Z=2, D(x)=3.92 g.cm-3, mu=160.2 cm-1, F(000)=1104, T=360 K, R(F)=0.042, R(F)w=0.040 for 231 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. In addition, some aspects of the high-temperature phase transition II-I were studied.