One of the many difficulties associated with reducing unburned hydrocarbon emissions during a cold-start from port-fuel injected engines is that the engine must be operated fuel rich to avoid partial bums and obtain suitable idle quality. If the dilution tolerance of the engine can be enhanced during the cold start, it would be possible to operate closer to stoichiometric or even fuel lean and thereby obtain significant reductions in hydrocarbon emissions. The overall objective of this study is to computationally explore the feasibility of extending the dilution limit of an engine during cold-start conditions by adding reformer gas to the inlet manifold of the engine. Because the gas mixture produced by reforming a hydrocarbon fuel contains large amounts of H-2, its addition to the hydrocarbon/air mixture holds the potential to enhance both the ignitability and flammability. n-Butane and iso-butane are chosen as surrogate hydrocarbon fuels because of their relevance to gasoline. In addition, comprehensive chemical kinetic models are available for these fuels. Numerical computations of the effect of reformer gas addition on the laminar flame speeds, lean flammability limits, and auto-ignition delay times of butane/air mixtures are performed with detailed chemistry and transport properties. Results show that addition of a first-stage reformer gas, H-2/CO/N-2 with volume percentages of 30/25/45, can increase the flame speed, extend the lean flammability limit, and reduce the ignition delay of butane/air mixtures over a wide pressure range. Although the second-stage reformer gas, H-2/CO2/N-2 with volume percentages of 45/20/35, contains more hydrogen than the first-stage reformer gas, its addition is shown to have an adverse effect on the burning rate due to the larger concentrations of N-2 and CO2 diluents. The present study demonstrates that the addition of a first-stage reformer gas, H-2/CO/N-2, in the intake of an engine would be a viable approach to reduce cold-start hydrocarbon emissions.