Previously a lab scale catalytic autothermal reformer (ATR) capable of operating at pressures from 6 to 50 bar was constructed and tested. The objective of the experimental program was to maximize H-2 production per mole of O-2 supplied (H-2(out)/O-2(in)). In this companion paper a 1-D, heterogeneous, numerical model is developed and tested for simulating the high pressure ATR. The effects of molar steam to carbon (S/C) and oxygen to carbon (O-2/C) ratios are studied and optimal operating conditions are identified for three system operating pressures; 6, 28 and 50 bar. Experimental optimal conditions and model results are compared and found to be in close agreement. The optimal conditions, however, predicted by the model at pressures of 28 and 50 bar have higher S/C ratios and produce higher H-2(out)/O-2(in) yields than the experimentally determined optimums. A sensitivity analysis consisting of 9 model parameters is also performed. The model is most sensitive to the activation energy of the two steam reforming reactions used in the model and the operating parameter O-2/C. (C) 2009 Elsevier B. V. All rights reserved.