This paper presents some insight into the role of sulfur, a well-known Fischer-Tropsch (FT) catalyst poison, regarding the selectivity of ethylene:ethane and C-2-product yield in a commercial, Fe-based, high-temperature, 2-phase FT process. It is well known that in these kind of processes, the level of sulfur allowed in the feed gas is normally very low (ppb range) and aggressively kept low due to the poisoning effects. The practice generally is to get sulfur levels down as close as possible to zero. The poisoning effects are well studied in research work and significant information exists in that regard. It is, e.g. known that the poisoning action of sulfur would typically increase selectivity towards more hydrogenated ethane relative to ethylene (the more attractive product). At high enough levels it would kill catalyst activity completely. This paper highlights the role of sulfur in a typical commercial operation. Data from a period of almost 2 years was used. It was found that under the prevailing actual commercial scale operating conditions, sulfur did not behave as a poison to the catalyst. However, its effect did not become completely negligible either. There were strong indications of sulfur promoting selectivity shifts towards more olefinic C-2-product. These results were also independently confirmed on a different set of data and a different kind of commercial reactor.