Numerical simulations were performed to determine the effect that varying the percent on-cell steam-methane reformation would have on the thermal, electrical, and mechanical performance of generic, planar solid oxide fuel cell stacks. The study was performed using three-dimensional model geometries for cross-, co-, and counterflow configuration stacks of 10×10- and 20×20-cm cell sizes. The analysis predicted the stress and temperature difference would he minimized for the 10×10-cm counter- and cross-flow stacks when 40 to 50% of the reformation :eaction occurred on the anode. Gross electrical power density was virtually unaffected by the reforming. The co-flow stack benefited most from the on-cell reforming and had the lowest anode stresses of the 20×20-cm stacks. The analyses also suggest that airflows associated with 15% air utilization may he required for cooling the larger (20×20-cm) stacks.