Cobalt supported on silica and titania catalysts were investigated by XPS, TPR, TPD, XRD and TEM in order to elucidate the Co/support interactions and their effect on the dispersion and reducibility of cobalt. The reducibility of cobalt was studied both with TPR experiments, in which the temperature is raised at a steady rate, and with XPS after in-situ reduction treatments at constant temperature. Silica-supported catalysts were prepared with various Co loadings (2-27.5 wt.%) and they exhibited significant structural and morphological differences. A significant Co/titania interaction was found, while no conclusive proof of any interaction was found for Co/silica. The degree of interaction between cobalt and the support affected not only the response of cobalt to reduction, but also its dispersion. In fact, cobalt spreads on titania during reduction and tends to sinter on silica.