The crystal structures of three phases Of CS5 H-3(SO4)(4)-xH(2)O-a low-temperature phase (LT phase), a room-temperature phase (RT2 phase), and an intermediate phase (RH phase)-have been determined from powder X-ray diffraction data measured at 90 - 450 K. The structures of the LT, RT2, and RH phases were orthorhombic Cmcm (a 6.304 angstrom; b= 10.388 angstrom; c=29.499 angstrom), triclinic P (1) over bar (a=6.248 A; b=6.187 angstrom; c- 14.833 angstrom; alpha=92.08 degrees; beta=88.86 degrees; gamma= 121.15 degrees), and trigonal P (3) over bar m1 (a=6.233 angstrom; c= 14.788 angstrom), respectively. The crystal structures of the hydrated and anhydrous forms in the usual room-temperature phase (RT phase) were also analyzed; reduction in the length of the a-axis was observed upon the loss of crystalline water, whereas the length of the c-axis remained constant. These results suggest that shrinkage in the a direction of the anhydrous form prevented reorientation of its SO4 tetrahedra, resulting in slower proton diffusion [K. Suzuki and S. Hayashi, Phys. Rev. B 74 (2006) 134303] than that in the hydrated form. These findings are consistent with a proton transport mechanism in which SO4 reorientation is the rate-determining step. (C) 2007 Elsevier B.V. All rights reserved.