Tungsten oxide (WO3) films were prepared by the sol-gel process, using 2,4-pentanedione (PTN) as an organic ligand. The effect of the ligand on the crystallization and crystal structure of the WO3 films was examined by Raman, IR and X-ray diffraction spectroscopies, and their optical properties were investigated in relation to the refractive index. Tungsten oxides prepared with PTN on a quartz glass substrate and a silicon wafer, and those prepared without PTN on a silicon wafer are amorphous when fired at 300 degrees C. The oxides crystallized when fired at temperatures between 300 and 500 degrees C, and the amounts of crystalline WO3 increased when the films were fired at 700 degrees C. Only cubic crystals of WO3 were formed selectively on the quartz glass substrates when the films prepared with PTN were fired at 500 and 700 degrees C. However, without PTN, a mixture of cubic and monoclinic crystals was formed under the same firing conditions. The WO3 film prepared with PTN and fired at 500 degrees C showed a lower value for the real part of the complex refractive index than that for the corresponding film prepared without PTN. The WO3 film prepared with PTN did not densify immediately after being fired, resulting in a lower value for the real part of the refractive index.