The coupling of CH (methylidyne) and CH2 (methylene), to form CHCH2 (vinyl), over Co and Ru surfaces has been studied with the nonlocal gradient-corrected density functional theory slab calculations. The results show that this reaction is slightly exothermic on Co while endothermic on Ru within a (2 x 2) surface unit cell. Transition states were isolated on both surfaces, and the reaction barriers were found to be 55.9 and 116.5 kJ/mol on Co and Ru, respectively. The structures of the transition state on the two metal surfaces are similar; both involve the formation of a multicentered bond.