The effect of the annealing temperature (T-a) on the optical, electrical and structural properties of the undoped cadmium oxide (CdO) thin films obtained by the sol-gel method, using a simple precursor solution, was studied. All the CdO films annealed in the range from 200 to 450 degrees C are polycrystalline with (111) preferential orientation and present high optical transmission > 85% for wavelengths above 500 rim. The resistivity decreases as T-a increases until it reaches a value of 6 x 10(-4) Omega cm for T-a = 350 degrees C. For higher temperatures the resistivity experiences a slight increase. Images obtained by atomic force microscopy show an evident incremental change of the aggregate size (clusters of grains) as T-a increases. The grain size also increases when T-a increases as observed in data calculated from X-ray measurements. (c) 2005 Elsevier B.V. All rights reserved.