A method for determining the refractive index, size, and concentration of silica nanoparticles used as cores in synthesis of gold nanoshells is described. The average refractive index of silica nanoparticles, n = 1.475 ± 0.005, is determined by a modified immersion method, which involves spectroturbidimetry data in immersion media (dimethyl sulfoxide + ethanol). Working calibrations are obtained, which allow one to determine the size and concentration of silica particles and, correspondingly, the concentration of gold nanoshells in final preparations from measured values of the wavelength exponent and optical density.