The effect of carbonyl sulfide (COS) impurity present in syngas on the performance of a traditional cobalt catalyst (0.5% Pt-25% Co/Al2O3) was investigated for Fischer-Tropsch synthesis (FTS) using a continuously stirred tank reactor (CSTR). Reports about the use of COS as sulfur carrier for the poisoning of cobalt catalysts for FTS are scarce. The S-poisoned and un-poisoned catalysts were tested at industrially relevant FTS conditions and also compared at the same CO conversion level in order to evaluate the effect of sulfur on catalyst activity and product selectivity. The COS (1.8 ppmv) addition caused a significant irreversible effect on the activity. At similar CO conversion level, the COS poisoned catalyst exhibited higher methane and C-2-C-4 selectivities and lower C-5+ selectivity compared to the clean or un-poisoned catalyst. The COS poisoned catalysts were characterized by X-ray adsorption near edge structure (XANES) analysis to find out the reason for the deactivation and altering of the product selectivities. The sulfur K-edge XANES results clearly indicate the formation of cobalt sulfide, which is known to enhance the hydrogenation ability. The olefin/paraffin ratio of the COS poisoned catalyst was found to be lower than that of the clean catalyst, providing further indication of the higher hydrogenation activity of the COS poisoned catalyst.