Radial temperature profiles during steam cracking result in radial nonuniformities in the product yields due to radial variations in the concentration of the radicals. The effect of using a 1‐D or a 2‐D reactor model on the calculated product yields is evaluated for the cracking of ethane. With a 2‐D reactor model the simulated ethylene yield decreases. Ethylene formed at the high‐temperature zone near the hot wall diffuses to the center where secondary reactions are favored, generating C₃ and C₄ olefins. This effect is confirmed by the calculation of a reactor of a Kellogg Millisecond Furnace. In this small‐diameter reactor the 1‐D behavior is more pronounced, resulting in higher ethylene yields at comparable conversions. The effect of the radial gradients on the coking rate calculated with a fundamental kinetic coking model based on elementary reaction steps is even more pronounced. Only when the coke model is coupled to a 2‐D reactor model, a good agreement with the reference data is observed. In order to obtain accurate simulation results the more detailed 2‐D reactor model is required, even if this increases the computational effort. © 2004 American Institute of Chemical Engineers AIChE J, 50: 173–183, 2004