The Fischer-Tropsch synthesis of hydrocarbons from carbon monoxide and hydrogen over cobalt catalysts is expected to become the key method for the future production of liquid fuels. Despite decades of research on the reaction mechanism, the state of the surface of the operating catalyst is still uncertain. Using in situ high-temperature high-pressure scanning tunneling microscopy, we have investigated the Fischer-Tropsch reaction over a Co(0001) single crystal model catalyst in the methanation limit. Atomically resolved images show that the surface does not transform into an oxide or carbide, but remains metallic under reaction conditions. The data are consistent with a mobile layer of reversibly adsorbed particles on the surface. The surface morphology under reaction conditions is unchanged from the surface in ultra-high vacuum. Widespread assumptions about a surface restructuring that seemed to explain the activity of cobalt-based Fischer-Tropsch catalysts are not confirmed. (C) 2014 Elsevier Inc. All rights reserved.