A new bicadmium-substituted vanadosilicate, [Cd(en)(2)](2)[(en)(2)Cd2Si8V12O40(OH)(8)(H2O)(0.5)]center dot 5H(2)O (1; en = ethylenediamine), had been hydrothermally synthesized and characterized. Structural analysis revealed that the kind of new [en)(2)Cd2Si8V12O40(OH)(8)(H2O)(0.5)](4-) polyoxoanionic cluster was derived from the classical {V18O42} cluster by replacing six {VO5} square pyramids with four {Si2O7} and two [Cd(en)](2+) groups. Notably, such mixed substitution of both main-group and transition metals in polyoxovanadates is much less developed. Furthermore, compound 1 displays efficient catalytic activity toward the selective oxidation of styrene to benzaldehyde with a conversion of 97% and a selectivity of 87% in 8 h.