The monoammonium salt of gamma-zirconium phosphate has been prepared, and its structure was solved by using X-ray powder diffraction data. The positions of the Zr and one of the P atoms were located from a Patterson map computed by using 29 unambiguously indexed reflections. Full pattern analysis using these two atoms and subsequent difference Fourier maps allowed the location of all the remaining atoms in the structure, including, the position of the nitrogen atom of the ammonium ion. Zr(PO4)(NH4HPO4) crystallizes in the monoclinic space group P2(1)/m with a 5.3284(4) Angstrom, b = 6.6217(2) Angstrom, c = 11.326(1) Angstrom, beta = 96.63(1)degrees, and Z = 2. The final agreement factors are R(wp) = 0.105, R(p) = 0.081, and R(F) = 0.043. The structure consists of metal phosphate layers forming the ab plane. The metal atoms and one of the phosphate groups are located nearly in the plane of this layer. All the oxygen atoms of this phosphate group are involved in metal coordination. The other phosphate group links two Zr atoms in the a-direction through two of its oxygen atoms. The remaining two oxygens, which constitute hydroxyl groups in the parent gamma-ZrP, are projected into the interlayer space and are hydrogen bonded to the ammonium ions. Additionally, there are direct interactions between adjacent layers in the form of hydrogen bonds among these oxygens. Each ammonium ion is shared by four oxygens from four different phosphate hydroxyl groups.