Properties of RbNO3 and CsNO3 in (1 - x)MeNO3-xSiO(2) (x = 0-0.9) nanocomposite solid electrolytes were studied by X-ray powder diffraction, differential scanning calorimetry methods and conductivity measurements. The used highly-dispersed silicas with narrow pore size distribution were different in their specific surface areas (13-580 m(2)/g) and pore size (R = 14-1000 Angstrom). The composite conductivity was shown to exceed that of individual salts by more than 1.5-4 orders of magnitude and to be maximum at x = 0.5-0.7. In nanocomposites based on alkali nitrates and silica the 'dimensional effect' was observed. The properties of composites depended markedly on pore size of silica. The optimum pore size of heterogeneous dopant was in a range of 35-100 Angstrom, where the most composite conductivity increase took place and thermodynamic and structural properties of ionic salts changed markedly. For composites based on these silicas the enthalpies of RbNO3 (CsNO3) phase transitions and melting decreased considerably. The ionic component became either partially or completely amorphous tin particular with x increase). The MeNO3 state changed slightly when the SiO2 pore size was 1000 Angstrom. In systems with pore size 14 (both crystalline low temperature RbNO3(IV) and amorphous salt were observed.