The electron microscopy technique was used to characterize the cobalt - silica interaction in a model catalyst at high temperature in air. The samples were in the form of thin cobalt films ( 1 and 4 nm thick) supported on an amorphous layer of SiO2. It was concluded that cobalt loading determines the evolution of the microstructure, morphology and phase composition. The spreading of the cobalt oxide phase and formation of cobalt silicate was established.