The effect of injection of hydrogen sulfide, an available inexpensive byproduct in Iran's petrochemical industry, on the rate of coke formation over chromium, iron, and stainless-steel (SS) coupons in thermal cracking of ethane was investigated. In a laboratory reactor, coke formation on the metal coupons was measured in the temperature range of 1098-1148 K after a fixed time of 900 s. Nitrogen was used as an inert diluent to exclude the concomitant coke oxidation in the commonplace steam-cracking process. The role of hydrogen sulfide was examined through an additive feeding policy either as a pure presulfidation of the coupons with a 200 ppmw concentration for 20 min or via continuous introduction of 25 and 50 ppmw amounts of hydrogen sulfide into the gas feed stream. The presulfidation reduced the rates of coke formation up to 20, 45, and 30% over Cr, Fe, and SS samples, respectively. However, the continuous injection of hydrogen sulfide showed a complex behavior depending upon the temperature, so that the rate of coke formation increased by 20-120% at higher temperatures but decreased by 25-30% at lower temperatures. The complex and contradictory behaviors of the H2S effect on the coke formation for the presulfidation and the continuous sulfidation scenarios were well-described through a simplified mechanism. An empirical model was developed to predict the rate of coke formation over the selected metal samples at given operating conditions and known H2S and ethylene concentrations. The model predictions were in good agreement with the experimental results.