0_7312 Cerium dioxide (CeO₂) is one of the most studied materials due to its non-toxicity and environmental friendliness.The characteristic of its electronic structure, high UV absorption capacity, high refractive index and It is one of the most critical rare earth materials due to its high transparency in the visible region. Tese films are known to be used in electrochromic devices (ECDs) as passive counter electrodes and in lithium-ion batteries due to their excellent charge density and reversibility properties. Several techniques were investigated to elaborate the CeO₂ thin films, such asmagnetron sputtering, Microwave-assisted hydrothermal, spin coating, electrochemicaldeposition, sol-gel, pulsed laser deposition, and spray pyrolysis technique. Among these methods, the sol–gel method is an attractive one due to its simplicity, safety, non-vacuum system of deposition, and inexpensive. Other advantages of this method are that it can be adapted easily for production of large-area films, and to get varying band gap materials during the deposition process.In this study, CeO₂ films using solutions prepared in different molarities were prepared by sol–gel process using a spin coating technique onto glass substrates. The precursor  solution was prepared by dissolving cerium sulfate [Ce(SO₄)₂] in deionized water at different molarities. The solution were stirred at 40^oC. The substrates were placed on the sample holder and were rotated at a speed of 2000 rpm for 30 s. Immediate drying after successive coating was done at 200^oC for 10 min. This process was repeated seven times. The films were finally annealed at 500^oC for 1 h in air for crystallization and phase formation. Surface morphology and crystalline structure of the films were investigated by field emission scanning electron microscopy (ZEISS ULTRA PLUS) and Xray diffractometer (BRUKER D2 Phaser),  respectively. All the films of XRD patterns presenting the formation of the cubic CeO₂ crystal structure (JCPDS reference 34-0394) having the polycrystalline nature. Diffraction peaks belong to (111), (200), (220), (311), (222), (400), (331) and (420) planes. As the solution molarity increased, the crystallization of the films improved. The lattice constants, crystalline size and preferred orientation of the films were calculated from X-ray data. The diffuse reflectance measurements of the films were performed by SHIMADZU 2450 spectrophotometer including an integrating sphere attachment with using barium sulfonate (BaSO₄) as reference. The values of some important optical parameters of the films are determined, such as refractive index, extinction coefficient and dielectric constants. The optical band gap of the films is determined.  AcknowledgementsThis work was supported by Eskisehir Technical University Commission of Scientific Research Projects under Grant No. 22ADP0278.