The use of gasoline fuels in compression ignition engines, with or without diesel pilots, has shown encouraging progress in engine efficiency and emissions. The dual fuel combustion of gasoline-diesel offers the flexibility of modulating the cylinder charge reactivity, but an accurate and reliable control over the ignition in the dual fuel applications is more challenging than in classical engines. In this work, the gasoline-diesel dual fuel operation is investigated on a single cylinder research engine. The effects of the intake boost, exhaust gas recirculation (EGR) rates, diesel/gasoline ratio, and diesel injection timing are studied in regard to the ignition control. The results indicate that at low load, a diesel pilot can improve the cylinder charge reactivity and reduce emissions of incomplete combustion products. However, as the engine load is raised, the port-injected gasoline tends to auto-ignite prior to the end of the compression stroke, and the NOx and soot emissions are significantly increased owing to the concurrence of the diesel injection event and the gasoline early auto-ignition. The test results also prove that such premature auto-ignition can be withheld by increasing the use of EGR and/or decreasing the gasoline fuelling amount. A reliable ignition control is identified as critical to enable the dual fuel combustion with ultralow NOx and soot.