The effect of 1.0 ppm H2S in the syngas feed on initial activity and selectivity of a 0.5%Pt-25%Co/Al2O3 catalyst was studied by comparing the catalyst performance under H2S and sulfur free conditions. The reaction tests were performed using a 1-L slurry phase reactor for 141-212 h under constant reaction conditions: 220 A degrees C, 2.0 MPa, H-2/CO = 2.0 and 6.0 Nl/g-cat/h. In the H2S poisoning test, an H2S in N-2 gas mixture was added to the syngas feed (1.0 ppm) after running the Fischer-Tropsch synthesis (FTS) reaction for ca. 6.0 h; as such, the impact of H2S on the initial deactivation of the cobalt catalyst (unsteady state) was successfully assessed. The used catalysts were characterized by XANES to investigate if Co-S surface species were formed during the deactivation. The initial deactivation under 1.0 ppm H2S condition was significantly higher (by 2.0-2.4 times) than that under clean conditions. CH4 selectivity increased substantially and C5+ selectivity decreased greatly with time regardless of whether H2S was added or not; however, the addition of H2S accelerated the changes in the hydrocarbon selectivities, which were likely caused by the sharp deactivation of the catalyst in the presence of H2S. After co-feeding the sulfur for 141 h, a comparison was made at similar conversions by adjusting space velocity; the sulfur pretreated cobalt catalysts favored heavier hydrocarbons (C5+) formation and suppressed lower hydrocarbon formation. The addition of H2S to the feed increased CO2 selectivity and the secondary reaction of 1-olefins. The XANES results revealed that surface species involving Co-S bonding formed on the cobalt catalyst after exposure to H2S during FTS. This was likely the primary reason for the abnormal selectivity trends observed during and after the deactivation of the catalyst by sulfur. This study points out a possible approach to increase the selectivity to heavy hydrocarbons by properly sulfiding the cobalt catalyst prior to the FTS reaction.