The carbonyl–ruthenium substituted undecatungstosilicate [α-SiW₁₁O₃₉Ru^II(CO)]⁶⁻ (1) was isolated as a caesium salt and successfully characterized by using ¹⁸³W and ¹³C NMR, elemental analysis, IR, UV-vis and cyclic voltammetry (CV). Polyanion 1 represents the first example of a metal–carbonyl moiety being fully incorporated into the polyoxometalate (POM). As a result, the Ru(CO) moiety became redox active and was reversibly oxidized to the one-electron ruthenium(III) derivative, [α-SiW₁₁O₃₉Ru^III(CO)]⁵⁻. This Ru^III(CO) moiety was unexpectedly stable in aqueous solution compared to the organo–ruthenium carbonyl derivatives and could be detected by using UV-vis and in situIR coupled with electrolysis. The oxidized ruthenium(III) derivative slowly released CO in aqueous solution, resulting in the aqua species [α-SiW₁₁O₃₉Ru^III(H₂O)]⁵⁻ and then the dimeric POM species by condensation. Furthermore, 1 could be converted to the corresponding aqua polyanion by photo-irradiation.