Silica-supported cobalt catalysts have been investigated by soft X-ray absorption techniques. Soft X-ray absorption spectra were collected at the Co L-II,L-III edge during in situ reduction of calcined samples in a stream of hydrogen in the temperature range between 300 and 650 degrees C. Using reference compounds, the structural transition occurring on Co3O4 clusters to give CoO species was established. Disappearance of the different features associated with Co2+ O-h symmetry was observed, probably due to disorder in the first coordination sphere of the cobalt atoms. For the interpretation of the spectra, numerical simulations based on the multiplet theory were carried out. Through these experiments we demonstrate the advantages of soft X-ray experiments in catalysis research involving 3d metals. (C) 2000 Academic Press.