The 1 : 1 : 2 potassium aluminum phosphate KAl(HPO 4 ) 2 • H 2 O has been synthesized and characterized by single-crystal X-ray diffraction. The compound crystallizes in the monoclinic space group P2 1 /c with Z.8؍ Crystal data: a)2(0.4001؍ pm, b)2(7.019؍ pm, c)3(4.5261؍ pm, ,°°)2(89.001؍ R g .440.0؍ A neutron-scattering experiment with subsequent refinement of the powder pattern of KAl(DPO 4 ) 2 • D 2 O elucidated the D atom positions in the deuterated compound. Crystal data: a؍ 1003.7(1) pm, b)1(5.019؍ pm, c)2(3.2261؍ pm, ؍ 100.97(1)°°, R wp .440.0؍ The structure consists of two different AlO 6 -octahedra and four different HPO 2؊ 4 tetrahedra. Corner sharing between these polyhedra creates a complicated threedimensional framework with K ؉ in cavities. Building units of that framework are Al phosphate chains that form various interconnections by corner sharing of polyhedra and hydrogen bonds. The hydrogen-bond donor and acceptor properties of the phosphates and water molecules are discussed along with their function in the structure. 31 P solid-state MAS spectra show only three iso values at ؊6.5, ؊10.7 and ؊12.6 ppm instead of the expected four different isotropic chemical shifts. The sideband system at ؊10.7 ppm has twice the intensity of each of the outer systems. Considering the surroundings of the four HPO 2؊ 4 tetrahedra it can be assumed that the first iso value results from P(4), the second from P(2) and P(3), and the third from P(1). Significant differences between a given value of the principal axes ii of the nuclear magnetic shielding tensors of the phosphorus atoms are observed only for 11 of P(4) being more deshielded and for 22 of P(1) being more shielded.