The ammonia method has been successfully used for preparing thermostable and well dispersed alumina-supported catalysts with a surface average size of cobalt particle D-s = 5.7 nm. The disproportionation reaction of CO over this Co/Al2O3 catalyst and a similar Co/SiO2 catalyst leads to the formation of carbon nanotubes demonstrating the same morphology. The amount of nanotubes over Co/Al2O3, however, is much larger than that obtained over Co/SiO2, because of a faster ageing in the latter solid. Similar support effects have already been reported for other catalytic reactions involving carbon oxides.