The adsorption of POMs on Rh/SiO 2 in water solvent under strongly reductive conditions was investigated. Aqueous solutions of α-Keggin type silicotungstate and silicovanadotungstates were mixed with Rh/SiO 2 at 393−473 K under 1 MPa of H 2 . Monovanadium-substituted silicotungstate, α-SiVW 11 O 40 5− (SiVW 11 ), was more readily adsorbed than nonsubstituted silicotungstate, α-SiW 12 O 40 4− (SiW 12 ). After adsorption at 433 K, SiVW 11 was desorbed from Rh/SiO 2 by oxidation with Br 2 water without change of the Keggin structure, as evidenced by 51 V NMR. Trivanadium-substituted silicotungstate, α-1,2,3-SiV 3 W 9 O 40 7− , was not stable, and the desorbed species from Rh/SiO 2 by oxidation with Br 2 did not maintain the Keggin structure. The very high temperature for adsorption (473 K) also led to the decomposition of the Keggin structure of SiVW 11 . An increase in the concentration of SiVW 11 in the liquid phase gave a saturation of the amount of desorbable SiVW 11 , up to five SiVW 11 anions per one Rh particle with a 3 nm size. The elemental analysis and W L 3 -edge extended X-ray absorption fine structure of Rh/SiO 2 after the adsorption of SiVW 11 showed that a part of SiVW 11 was decomposed and irreversibly adsorbed as metallic W species incorporated into the surface of Rh metal particles. The amount of decomposed SiVW 11 was almost the same as that of SiVW 11 adsorbed as the original Keggin structure. The desorbable SiVW 11 was probably bonded on the W atom incorporated on the Rh metal particles as the two-electron-reduced form (α-SiV III W 11 O 40 7− ).