A new copper(II) molybdenum(VI) arsenate(III), (C 5 H 5 NH) 2 (H 3 O) 2 [(CuO 6 )Mo 6 O 1 8 (As 3 O 3 ) 2 ], has been hydrothermally synthesized and characterized by single crystal X-ray diffraction and TG analysis. The compound crystallizes in the monoclinic space group P2 1 /n with a = 9.303(2), b = 20.731(4), c = 12.617(3) 9, β = 104.17(3) o , V = 2359.3(8) 9 3 , Z = 2 and R1 (wR2) = 0.0296(0.0683). The structure is composed of pyridinium cations, proton hydrates and [(CuO 6 )Mo 6 O 1 8 (As 3 O 3 ) 2 ] 4 - polyanions. The polyanion framework derives from the Anderson type; the central octahedron is filled up by copper(II) and is capped on both sides by a cyclic As 3 O 6 group.