Possible intermediates in the formation of carbon deposits in cobalt -based Fischer−Tropsch synthesis were investigated using DFT. Larger carbon clusters are more stable than adsorbed atomic carbon, with linear carbon structures being (slightly) more stable than branched structures. The formation of aromatic structures further stabilizes the carbon cluster. A lower estimate for the barrier of diffusion of atomic carbon on flat FCC-Co(111) was estimated to be ca. 42 kJ/mol implying relative high mobility under typical Fischer−Tropsch conditions. A reaction pathway for the formation of graphene on a flat surface is proposed.