Silicon-cobalt oxide thin films were prepared by the dipping sol-gel process. Samples with different number of dipping-annealing cycles were prepared. Some data regarding the precursor sol are given from small angle X-ray scattering characterization. Composition, structure, surface morphology and optical properties are obtained from X-ray diffraction, reflectance, transmittance, FTIR, scanning electron microscopy and EDX spectroscopy measurements. The silicon-cobalt oxide thin films prepared in this work are mostly amorphous. They have a high absorption coefficient in the visible and infrared regions. A refractive index from 2.15 to 1.79 (at 1200 nm wavelength), and a hand gap between 3.73 and 3.68 eV with increasing film thickness were measured in the films. Sol-gel prepared Si-Co oxide thin films could be well suited for use in photothermal solar collectors. (C) 2002 Elsevier Science B.V. All rights reserved.