Nanostructured thin films of cerium dioxide have been prepared on single-crystalline silicon substrates by ultrasonic spray pyrolysis using cerium acetylacetonate as a metal–organic precursor dissolved in anhydrous methanol and acetic acid as an additive. The morphology, structure, optical index, and electrical properties were studied by X-ray diffraction, scanning electron microscopy, atomic force microscopy, ellipsometry, and impedance spectroscopy. The use of additives is very important to obtain crack-free films. The substrate temperature and flow rate was optimized for obtaining smooth (Ra<0.4 nm), dense (n>2), and homogeneous nanocrystalline films with grain sizes as small as 10 nm. The influence of thermal annealing on the structural properties of films was studied. The low activation energy calculated for total conductivity (0.133 eV) is attributed to the nanometric size of the grains.