We report on MWNTs synthesised using the aerosol method. The catalyst particles are formed by dissolving metal salts in organic solvents. This solution is sprayed as aerosols into a furnace. The active catalyst is formed in-situ, using H-2 as carrier gas and as reducing agent of the metal ions. This synthesis route gives possibilities for using a uniquely wide range of metal catalyst precursors and carbon sources. Using this approach, the size of the active catalyst particles can be calculated. MWNTs, prepared using Co(NO3)(2)(.)6H(2)O as precursor and tetrahydrofuran, serving as solvent and carbon source demonstrate the reliability of these predictions. (C) 2003 Elsevier B.V. All rights reserved.