A nickel molybdenum phosphate, (NH 3 CH 2 CH 2 NH 3 ) 4 · (NH 3 CH 2 CH 2 NH 2 ) · Na · [Ni 2 Mo 12 O 30 (PO 4 )(HPO 4 ) 4 (H 2 PO 4 ) 3 ] · 6H 2 O, involving molybdenum present in V oxidation, has been hydrothermally synthesized and structurally characterized by single crystal X-ray diffraction. Deep brown-red crystals are formed in the triclinic system, space group P1, a=12.011(2), b=14.612(3), c=21.252(4) Å, α=80.54(2)°, β=83.10(2)°, γ=76.29(2)°, V=3561.4(12) Å 3 , Z=2, λ(MoKα)=0.71073 Å (R(F)=0.0529 for 9880 reflections). Data were collected on a Siemens P4 diffractometer at 20°C in the range of 1.75°<θ<23.02° using the ω-scan technique. The structure was solved by direct methods using the program SHELXTL-93 and refined with the method of full-matrix least-squares on F 2 . The structure of the title compound may be considered to be two [Mo 6 O 15 (HPO 4 )(H 2 PO 4 ) 3 ] 5− units bonded together with a nickel atom, although several P–O groups are not protonated on account of coordination with a Na + cation. The one-dimensional tunnels were formed in the solid of the title compound. A probe reaction of the oxidation of acetaldehyde with H 2 O 2 using the title compound as catalyst was carried out in a liquid– solid system, showing that the title compound had high catalytic activity in the reaction.