AbstractA model has been developed for the forced feed concentration cycling of a Fischer‐Tropsch synthesis reactor containing an iron catalyst promoted with potassium and copper. It was based on an initiation sequence proposed by Vannice (1975) for methanation, a propagation sequence using the CO‐insertion mechanism, and termination step based on the desorption of each n‐mer, including CH4. Only surface adsorption, surface reaction and desorption steps were considered; no storage in the catalyst bulk was assumed.Agreement of the model and experimental results permits interpretation of steady‐state, as well as the cycling and step‐change rate measurements. The model shows that hydrogen is involved in the rate determining step, and further that, except for methane, the results of hydrocarbon production from steady‐state, step‐change, and periodic experiments could all be reconciled for the results from C2 to C9. Failure of the model properly to predict rates of methane formation during cycling operation is attributed to changing carbidization of the catalyst.