We describe a polymer electrolyte fuel cell model emphasizing operation on hydrocarbon reformate, i.e., the anode feed stream consists of dry H-2 concentrations as low as 40%, inlet CO levels of 10-100 ppm, and hydrogen fuel utilization as high as 90%. Refinements of interfacial kinetics equations used in our previous work on CO effects in H-2 anodes have yielded a better quantitative ft to the measured dependence of voltage loss on inlet CO level [in Electrode Materials and Processes for Energy Conversion and Storage, J. McBreen, S. Mukerjee, and S. Srinivasan, Editors, PV 97-13, pp. 15-24, The Electrochemical Society Proceedings Series, Pennington, NJ (1997)]. We calculate anode potential losses by coupling such interfacial kinetic processes to reactant diffusion limitations and ionic resistance in the catalyst layer, and by accounting for the drop in local hydrogen concentration along the flow channel due to significant fuel utilization. As a result of internal readjustment of cell overpotentials when hydrogen concentration drops along the flow channel, we show that loss of current, or power, under the realistic condition of constant cell voltage is smaller than loss of current at constant anode potential. We show that voltage losses associated with CO poisoning are significantly amplified with diluted hydrogen feed streams and particularly so under high fuel utilization. We make projections on improvements required, qualitative and quantitative, in the physical parameters of the anode catalyst surface chemistry to significantly improve "CO tolerance". (C) 2000 The Electrochemical Society. S0013-4651(00)03-119-0. All rights reserved.