The U.S. Department of Energy funded Co-Optimization of Fuels and Engines initiative (Co-Optima) aims to simultaneously transform both transportation fuels and engines to maximize performance and energy efficiency. Researchers from across the DOE national laboratory system are working within Co-Optima to develop merit functions for evaluating the impact of fuel formulations on the performance of advanced engines. The merit functions relate overall engine efficiency to specific measurable fuel properties and will serve as key tools in the fuel/engine co-optimization process. This work focused on developing a term for the Co-Optima light-duty boosted spark ignition (SI) engine merit function that captures the effects of fuel composition on emissions control system performance and overall engine efficiency. For stoichiometric light-duty SI engines, the majority of NOx, NMOG, and CO emissions occur during cold start, before the three-way catalyst (TWC) has reached its “light-off” temperature. This light-off temperature depends on the exhaust composition, which changes with fuel formulation. Thus, the time to achieve light-off, and therefore the cold start emissions, depends on fuel composition. Since the time to reach light-off must be minimized to meet emissions regulations, modern vehicles employ cold start strategies that intentionally dump fuel energy into the engine exhaust to heat the TWC above the light-off temperature as quickly as possible. The fuel penalty associated with catalyst heating during cold start provides a link between fuel properties, emissions performance, and engine efficiency. This paper will describe the derivation of the Co-Optima SI engine merit function term for emissions control, which is based on the cold start fuel penalty for catalyst heating and the catalytic light-off temperature for a particular fuel composition. It will also discuss the process used for extracting cold start fuel penalties from chassis dynamometer data sets previously collected at ORNL.