The solubility of hexafluoridosilicate salts and acid aqueous solutions {yH(2)[SiF6] + (1 - y)M-2[SiF6]}(aq) with Na+, K+, and NH4+ was experimentally determined at various ionic strength fractions (y) with y = 0.20, 0.50, and 0.80 at the temperature 353.15 K. The precipitated solid phases obtained by filtration were characterized by X-ray diffraction, which show only the presence of a single crystalline form for each ternary system at various (y) values, Na-2[SiF6], K-2[SiF6], or (NH4)(2)[SiF6], respectively. To find the solubility, the thermodynamic properties of these ternary systems are required, based on the measured activity of water. Then, the water activity measurements for the mixed systems were done at various (y) values with y = 0.20, 0.50, and 0.80 from diluted to the saturated solutions, using the hygrometric method at T = 353.15 K. The related thermodynamic properties of hexafluoridosilicate salts and acid aqueous solutions were also determined using Pitzers ion-interaction approach. The new ion-interaction parameters theta((H,M)) and Psi((H,M,[SiF6])), with M = Na+, K+, or NH4+, of the ternary systems were evaluated and employed with the interaction binary parameters to estimate the solute activity and the osmotic coefficients of water for these aqueous solutions at T = 353.15 K. The calculated solubility of mixed systems is compared with that obtained experimentally.