A new 3-dimensional zirconium hydrogen phosphate. tau-Zr(HPO4)(2), was prepared by hydrothermal syntheses. The preparation, characterization, and determination of the crystal structure of this new material are reported. The structure was solved from single-crystal synchrotron diffraction on a 25 x 25 x 50 mu m crystal. The position of the hydrogen atom was found using high-resolution neutron powder diffraction data : The unit cell is tetragonal, space group l4(1)cd (No. 110), with lattice parameters a = 11.259(1) Angstrom, c = 10.764(1) Angstrom, and Z = 8. The structure can be described as a 3-dimensional framework built from ZrO6 octahedra bound together by hydrogen phosphate groups (PO3(OH)). The OH groups form a hydrogen-bonded spiral along the 4(1) axes, with each hydrogen phosphate both donating and accepting hydrogen bonds. The thermal transformations of tau-Zr(HPO4)(2) were studied using in-situ temperature-resolved powder diffraction experiments. Hereby the existence of two new phases was discovered. The new phases have been indexed and the unit cell parameters are as follows, intermediate I : a = 8.172(1), b = 7.691(1), and c = 10.794(2) Angstrom. Intermediate II : a = 8.283(2), b = 6.620(2), and c = 10.643(2) Angstrom.