Effects of fuel/air equivalence ratio variations (Φ = 1.0±0.02) on engine-out and catalyst-out hydrocarbon (HC) mass and speciated emissions were measured under simulated cold-start conditions in order to suggest ways to optimize the engine-controls-catalyst system for minimum HC mass emissions and specific reactivity. A single-cylinder engine (installed in a temperature-controlled room and using commercial-grade gasoline) is run under controlled steady-state conditions (at 24 °C or -7 °C) which simulate cold starting. Speciated and total hydrocarbon emissions are measured from engine-out exhaust samples and from samples taken after an oven-temperature-controlled catalyst (either a fresh platinum/rhodium production catalyst, a 50,000 mile vehicle-aged catalyst, or a ceramic brick with standard washcoat containing no noble metal). Changes in engine fuel/air equivalence ratio (Φ = 1.0±0.02) have a small effect on engine-out HC mass emissions (± 10 %) and specific reactivity (0 - 2%). However, changing Φ from 1.02 (slightly rich) to 0.98 (slightly lean) has a large effect on catalyst performance - decreasing post-catalyst ozone forming potential values by a factor of 36 for a fresh catalyst and a factor of 6 for an aged catalyst.