The effect of Pt and Ru promoters on the deactivation of Co catalyst by carbon deposition during CO hydrogenation is investigated using Density Functional Theory (DFT). The barriers for diffusion of C on the catalyst surface were found to be low on the promoted and unpromoted surfaces suggesting that C diffusion on these surfaces could occur easily facilitating coupling of C with other adsorbed species. The promoters did not significantly alter the barrier for diffusion of C to the subsurface. Both Ru and Pt reduced the barrier for C H coupling and increased the barriers for C C and C C C coupling. Most of the C structures (both finite and infinite as well ring compounds) on Pt promoted surface had lower stability than the C structures on the unpromoted surface. Ru reduced the stability of the larger C structures. The C structures on Pt promoted surface were less stable than C structures on Ru promoted surface. These results give insights into effects of Pt and Ru promoters on C deposition pathways that could occur on Co catalysts during FTS reactions. (C) 2013 Elsevier B.V. All rights reserved.