Cobalt-silicon mixed oxides with Co/Si ratio of 10/90 (10Co), 20/80 (20Co) and 30/70 (30Co) were prepared by a modified sol-gel method. The materials treated in air at 400 and 600 degrees C were characterized by SEM and TPR/TPO techniques. TPR measurements showed that in all samples only a fraction of Co was present as Co3O4 and as amorphous silicate and was reducible by H-2 within 800 degrees C, while a part was not reducible under TPR conditions. The fraction of Co not reducible decreased with increasing Co content. A TPO/TPR cycle gave rise to an increase of the fraction of not reducible Co. (C) 2008 Elsevier B.V. All rights reserved.