New regulations from the state of California have established, for the first time, reactivity-based exhaust emissions standards for new vehicles and require that any clean alternative fuels needed by these vehicles be made available. Contained in these regulations are provisions for “reactivity adjustment factors” which will provide credit for vehicles which run on reformulated gasoline. The question arises: given two fuels of different chemical composition, but both meeting the criteria for CA Phase 2 gasoline (reformulated gasoline), how different might the specific reactivity of the exhaust hydrocarbons be? In this study we explored this question by examining the engine-out HC emissions from a single-cylinder version of the 5.4 L modular truck engine run on two different CA Phase 2 fuels. The results were for steady-state operation under five different operating conditions: world wide mapping point (1500 rpm, MBT, 3.8 bar IMEP) at three fuel/air equivalence ratios (ϕ = 0.9, 1.0, 1.15); a retarded spark timing (MBT - 12°) at ϕ = 0.9; and a higher speed (2500 rpm, MBT, 4.14 bar IMEP) at ϕ = 0.9. This matrix was designed to test a variety of operating conditions that may be encountered in actual vehicle use. Both fuels were run at least twice at each operating condition. The exhaust was sampled and analyzed for both total and speciated hydrocarbons. MTBE was also measured, but no analyses were carried out for other oxygenated organic compounds. For each sample the specific reactivity of the exhaust was calculated. Based on these engine-out exhaust emissions data, the specific reactivities of the non-methane hydrocarbon emissions from the two California Phase 2 certification fuels are identical to within our estimated statistical uncertainty of 1-2% for all 5 engine operating conditions.