The poisoning of a Co(0 0 0 1) model catalyst for the Fischer-Tropsch reaction by sulfur was investigated by in situ scanning tunneling microscopy (STM). Experiments were performed under syngas at pressures around 10 mbar and at sample temperatures around 500 K under methanation conditions. After the reaction experiments, the surface was analyzed by X-ray photoelectron spectroscopy. Mass spectrometry showed that the sample was poisoned when hydrogen with <0.5 ppm of sulfur was used in the syngas. STM images recorded under these conditions showed a strong restructuring of the surface into small terraces. The terraces were fully covered by a stable surface phase, displaying small ordered areas with (2 x 2) and (root 19 x root 19)R23.4 degrees structures. The observations contrast with the flat terraces covered by a mobile adsorption layer when sulfur-free hydrogen was used. The poisoned surface state is interpreted as a mixed sulfur/carbon layer that additionally involves a reconstruction of the Co(0 0 0 1) surface. (C) 2015 Elsevier Inc. All rights reserved.