The equilibrium adsorption on activated carbon of tungstophosphoric or tungstosilicic acid, both from solutions in ethanol-water 50% v/v, was studied at 20degreesC. It was found that the adsorption strength was higher for the former acid than it was for the latter. The solutions were characterized by UV-visible and 31 P nuclear magnetic resonance spectroscopies and the results indicated that the main species present are the undegraded anions of the acids. This behavior is opposite that observed in aqueous solutions in which, depending on the heteropolyacid considered, different transformations take place. The solids obtained, which were dried at 20degreesC and studied by X-ray diffraction, contain highly dispersed noncrystalline adsorbed species. By 31 P nuclear magnetic resonance and Fourier transform infrared spectroscopies on the solids, it was found that such species are the anions [PW12 O40 ]3- and [SiW12 O40 ]4- with undegraded structure for the tungstophosphoric and tungstosilicic acids, respectively. Besides, it was proved that the thermal behavior up to 365degreesC of the adsorbed species is similar to that of their corresponding bulk acids, i.e., neither tungstophosphoric nor tungstosilicic acid undergoes degradation.