Yttrium oxide thin films are deposited on silicon substrates using the ultrasonic spray pyrolysis technique from the thermal decomposition of a beta-diketonate, yttrium acetylacetonate (Y(acac)(3)). The decomposition of Y(acac)(3) was studied by thermogravimetry, differential scanning calorimetry, mass spectrometry, and infrared spectroscopy. It was found that a beta-diketone ligand is lost during the initial steps of decomposition of the Y(acac)(3). The rest of the complex is then dissociated or degraded partially until Y2O3 is obtained in the final step with the presence of carbon related residues. Then the Y(acac)(3) was used to synthesize Y2O3 thin films using the spray pyrolysis technique. The films were deposited on silicon substrates at temperatures in the range of 400-550 degrees C. The films were characterized by ellipsometry, infrared spectroscopy, atomic force microscopy, and X-ray diffraction. The films presented a low surface roughness with an index of refraction close to 1.8. The crystalline structure of the films depended on the substrate temperature; films deposited at 400 degrees C were mainly amorphous, but higher deposition temperatures (450-550 degrees C), resulted in polycrystalline with a cubic crystalline phase.