Dual-fuel combustion strategies combining a premixed charge of natural gas and a pilot injection of diesel fuel offer the potential to reduce CO2 emissions as a result of the high Hydrogen/Carbon (H/C) ratio of methane gas. Moreover, the high octane number of methane means that dual-fuel combustion strategies can be employed on compression ignition engines without the need to vary the engine compression ratio, thereby significantly reducing the cost of engine hardware modifications. The aim of this investigation is to explore the fundamental combustion phenomena occurring when methane is ignited with a pilot injection of diesel fuel. Experiments were performed on a single-cylinder optical research engine which is typical of modern, light-duty diesel engines. A high-speed digital camera recorded time-resolved combustion luminosity and an intensified CCD camera was used for single-cycle OH*chemiluminescence imaging. Experiments were performed for a wide range of equivalence ratios of the premixed charge. At low equivalence ratios, optical engine results revealed that combustion of the premixed charge of methane gas was dominated by spray entrainment and mixture stratification of diesel fuel. At higher equivalence ratios particularly close to stoichiometry, time-resolved natural luminosity images revealed significant modifications in combustion behavior indicating some evidence of flame propagation. Corresponding rates of heat release support the optical measurements in terms of revealing a significant impact on combustion following an increase of equivalence ratio. PLIF-tracer experiments were also performed in order to investigate the influence of in-cylinder fuel distribution on dual-fuel ignition.