Transport. thermal and structural properties of (1 -x)(NH4)(3)H(SO4)(2)-xSiO(2) composite solid electrolytes (x = 0-0.8) were investigated, and an increase in (NH4)(3)H(SO4)(2)(II) low temperature phase conductivity was shown, The dependence of the low temperature conductivity on x has a smooth maximum with values five to six times higher than that of the pure polycrystalline salt and two orders of magnitude higher than that of single crystals. The (NH4)(3)H(SO4)(2) phase transition temperature in the composites decreases by 20 K with an increase of SiO2 content from 0 to 0.7, The observed jump in conductivity that coincides with the phase transition becomes more diffuse and disappears at x = 0.8. The thermal stability of (NH4)(3)H(SO4)(2) in such composites increases markedly as compared with the pure salt.