SiO2 coatings containing cobalt were prepared on glass substrates using the sol-gel method. It has been found that in coatings with a Si to Co ratio of 1.3, a layer of Co3O4 is formed at the free surface of the coating upon thermal annealings in air. The properties of the coatings were studied using optical, x-ray and Auger depth profile measurements. The thermal annealings in the temperature range of 300 to 500 oC, in steps of 50 oC, were performed for 10 min. Some samples were subjected to an isothermal annealing at 400 °C for different times from 10 to 210 min. From the x-ray diffraction patterns the cubic spinel structure of Co3O4 was detected after the thermal treatments. The optical reflection and transmission spectra for each annealing temperature and annealing time, are described with an air-Co3O4-SiO2:Co2+-substrate system. From this, the cobalt oxide thickness was obtained as a function of the annealing temperature and annealing time. From the Arrhenius plots, in the temperature range studied, it was found that the activation energy for the growth of the cobalt oxide layer is 0.41 eV. The layer thickness follows a parabolic behavior with time, which suggests a diffusion-controlled process. The Auger depth profiles obtained from a sample annealed at 500 °C confirm the optical model used.