The coking of stainless steel 304H alloy (SS304H) has been investigated in ethane-steam mixtures in the presence/absence of a few ppm of H 2 S at 700uC. In this, first detailed study of the influence of H 2 S on carbon morphology on SS304H, we show that the addition of H 2 S greatly altered the carbon morphology, in the long-term experiments (90 h). For shorter exposure times (4 h), carbon formation decreased as H 2 S concentration was increased. This improved carbon tolerance of SS304H is likely due to adsorption of sulphfur on its surface. Addition of H 2 S did not change ethane cracking temperature or product distribution.