The long term effect of up to 10ppm H2S was studied for Fischer-Tropsch conversion of syngas by catalytic testing at 240 degrees C, 5bar, H-2/CO=2.1 and GHSV=2400 Nml/g(cat) h. Sulphur was dosed after similar to 300 or similar to 1000h time on stream and the effect was monitored using on-line GC. The activity declined, and the effect correlated to the concentration of H2S in the feed. However, dosing after similar to 1000h caused a stronger effect than dosing after similar to 300h. The effects of sulphur are significant with respect to operational risks and mitigation but are substantially less severe than for a standard Co-based catalyst operated at 20bar for wax production. The spent catalyst consisted of a mixture of cubic (Co, Mn)O, hexagonal Co, Co2C and sulphurous deposits; mainly MnSO4. It could not be concluded that sulphur had a direct effect on product selectivity, but it may have impacted water-gas-shift activity, and sudden changes in shift activity was found to correlate to changes in hydrocarbon selectivity. [GRAPHICS] .