An approach is presented for determining an upper bound on the yield of a catalytic process, which allows for variations in the catalytic chemistry. Scaling and thermodynamic arguments are used to set parameters of an elementary step surface mechanism at values resulting in optimal yields, subject only to physical constraints. Remaining unknowns are treated as independent variables and varied over a broad range. The result is a set of thermodynamically consistent mechanisms with optimal kinetics that can be incorporated into reactor-transport models to generate yield trajectories. With this approach, an upper bound on the yield for oxidative coupling of methane (OCM) was computed. Results show that even with optimal surface chemistry, limits exist on the attainable yield. Surface energetics necessary for superior OCM performance were identified and the origins of these requirements elucidated. The resulting upper bound on OCM yield under conventional, packed-bed, continuous-feed operation was found to be 28%. (C) 2003 Elsevier Inc. All rights reserved.