The transient performance of an autothermal reformer with methane as the feedstock fuel is studied by modeling approach. The reformer is of cylindrical shape with 60 mm in diameter and 350 mm, in length filled with spherical Ni catalyst with 1.75 nun in diameter. A two-dimensional heterogeneous reformer model has been successfully developed to predict the dynamic performance of the reformer subjected to a step-up, a step-down and a sudden change of square-pulse flow rate. The model includes four main simultaneous chemical kinetic reactions involving six species with detailed kinetic conversion and heat and mass transfer phenomena in the reformer. Results showed that there are serious overshoot/undershoot of performance related parameters in the transients, whose values are quite different from those under the steady state conditions. (c) 2005 Elsevier Ltd. All rights reserved.