The influence of porous structure and addition of citric acid on the formation of ceria particles inside the SBA-15 pores is studied using nitrogen adsorption-desorption at −196 °C, XRD, UV-visible spectroscopy, TEM and TPR-H2 methods. The use of pores of SBA-15 as nanoreactors provides the formation of CeO2 nanoparticles with sizes below 6 nm. According to XRD and TEM results, the addition of citric acid during the impregnation allows obtaining the CeO2 particles with sizes about 3 nm, while the agglomerates of large СеО2 particles are formed on the surface of the SBA-15, when citric acid is not added. Small sizes of ceria particles (3 nm) cause an enhanced reducibility (TPR-H2), high defectiveness of the particles and red shift in the UV-vis adsorption spectra. These properties may improve the catalytic and photocatalytic activity of the CeO2/SBA-15-based catalysts in deep and selective oxidation of organic compounds.