NH(4)Fe(HPO(4))(2) and its deuterated form have been synthesized as monophasic polycrystalline materials. Their crystal structures, including hydrogen positions, were determined by Rietveld refinement and Fourier synthesis using constant-wavelength neutron powder diffraction data. In addition, the thermal decomposition of NH(4)Fe(HPO(4))(2) was found to give mixtures of Fe(4)(P(2)O(7))(3) and Fe(PO(3))(3)via NH(4)FeP(2)O(7) formation, the crystal structure of which has also been refined from X-ray powder diffraction data.