Product distributions of iron based catalyst represented by double ASF distributions characterized by two growth probabilities. Based on enol mechanism and double ASF distributions the complete set of elementary reactions is given. By using mechanistic kinetic studies of Fischer–Tropsch reaction, the chain growth probabilities (α 1 and α 2 ) for double ASF distributions are formulated. Fe–Cu–La catalyst was prepared by co-precipitation procedure, and steady-state FTS reaction rates were measured in a continuous spinning basket reactor. The experimental results for low carbon monoxide conversions have been satisfactorily fit with the double ASF model. But for higher carbon monoxide conversions deviate substantially because the FTS reaction highly depends on the hydrogen formed by the WGS as the carbon monoxide conversion increases.