The catalytic growth of filamentous carbon, from the decomposition of ethylene, has been studied over the temperature range 673-898 K using Ni/SiO2 doped with LiBr and CsBr. Promotion/inhibition effects due to the presence of these adatoms have been assessed; an optimum alakali metal loading is reported for Li, while the presence of Cs inhibited the reaction at all temperatures. Transmission electron microscopy has been used to characterise the ordered nature of the filamentous carbon growth.