Results of a temperature-dependent crystallographic study of the solid state protonic conductor alpha-Zr(HPO4)(2) . H2O are presented. Lattice parameters of this monohydrate and of its dehydrated phases, alpha-Zr(HPO4)(2) and beta-Zr(HPO4)(2), have been determined in the temperature range from 20 K to 650 K using X-ray and synchrotron powder diffraction. Neutron data of the protonated and of the deuterated monohydrates were refined by the Rietveld method, paying particular attention to the H- and D-sites, respectively, The dehydration process has been studied by comparing temperature-dependent coherent and incoherent neutron scattering intensities with X-ray powder diffraction and DTA/TG measurements. Proton conduction mechanisms are discussed in the light of these results.