Preparation and investigation of cerium-based nanocomposites are of scientific and practical interest due to their unique catalytic, optical, and adsorption properties. CeO₂/SiO₂ nanocomposite was synthesized via thermolysis of amorphized CeCl₃∙7H₂O on a fumed silica surface at 400–800 °С. According to XRD and TG data, the phase transition from crystalline to amorphous state during the preparation of the initial composite and a change in the thermal dehydration process of CeCl₃∙7H₂O on the silica surface are observed in comparison with individual cerium chloride at 100–250 °C. A cubic cerium dioxide with a crystallite size of 4 nm is formed in the silica matrix upon the thermolysis of the blend at 400 °С. With increasing temperature from 400 to 800 °С, a slight increase in the ceria crystallite size is observed. According to SEM data, the single-domain spherical-like CeO₂ nanoparticles (4–11 nm) are uniformly distributed in the amorphous silica matrix in the composite synthesized at 400 °С. The size distribution of CeO₂ estimated from XRD data is in agreement with HRTEM data. This study shows a pathway to control the formation of ceria nanocrystallites in the silica matrix that may be of interest from a practical point of view.