A conductivity cell is used to measure the thermal diffusion of aqueous silico-12-tungstic acid (H4SiW12O40). This system shows one of the largest Sorer effects ever measured for an electrolyte : a 2.2% change in the concentration of silicotungstic acid per degree. Thermodynamic considerations suggest that even larger thermal separations can be achieved by using a supporting electrolyte. This prediction is tested by measuring the thermal diffusion of silicotungstic acid in aqueous perchloric acid solutions. In the presence of a large excess of perchloric acid, a 1 K temperature difference produces a remarkable 9% (+/-1%) change in the concentration of silicotungstic acid. Isothermal diffusion coefficients are reported for aqueous solutions of silicotungstic and perchloric acids at 25 degrees C.