The production of carbon nanotubes by the chemical catalytic vapour deposition, CCVD, process was examined over iron, cobalt, and a mixture of iron and cobalt supported on alumina catalysts synthesized by a one step sol–gel process. The catalysts were synthesized from several metal precursors, iron nitrate, cobalt and iron acetylacetonate, and cobalt acetate. Ethylene was used as the carbon source. The Co/Al₂O₃ catalysts showed better activity and selectivity in carbon nanotubes synthesis than Fe/Al₂O₃ and Fe–Co/Al₂O₃ catalysts. The carbon deposit was found by TEM analysis to be rich in carbon nanotubes in the case of Co/Al₂O₃ but to be very poor in the case of the Fe–Co/Al₂O₃ catalysts. The catalysts were characterized by TEM, XRD, and nitrogen adsorption. It was shown that iron and cobalt are in oxide form. Metal–support interactions and metal oxide particle size are influenced by the nature of the precursor and this nature is an important factor for the activity and selectivity of the catalysts. Moreover, a correlation has been found between the metal oxide particle sizes, the diameter of the carbon nanotubes, and the catalytic activity.