The potassium salt of the new mixed-polyoxotungstate anion [ Eu3( H, O), (SbW, O, ) (W50,&]l8-(1) has been prepared from WO, , Sb, O, , Eu(N0,),*6H20, and KOH in water and isolated in crystalline form as K, , H, [ Eu, ( H, O), (SbW, O, ) (W,O,,),]*25.5 H, O. It crystallizes in the monoclinic space group C2/m, with a = 30.250(7), b = 18.568(5& c = 22.101 (6) A, p = 109.19(8)", and, core is co-ordinated by a B-type a-SbWgO, unit and three W,018 units with tetrahedral conformation. Each Eu3+ in the core exhibits eight-co-ordination by oxygen atoms belonging to H20, SbWgO, , and W,018 units. The intramolecular energy transfer from the 0-W charge-transfer levels for the polyoxotungstate crystalline lattices to the emitting 50, level of Eu3+ takes place efficiently at least over 6.9 A which is the largest distance between W and Eu atoms in the anion. A comparison of the lifetime and quantum yield of the emission among (1 ), [Eu(W,0,,), Ig-, and [Eu(SiW,,O,),]l3-indicates that the hopping of a Cr electron between WO, octahedra is the predominant deactivation channel of the O-+W charge-transfer levels, which reduces drastically the communication with the excited levels of Eu3+.