In a dual-fuel (DF) combustion process, the ignition of the main fuel plays a crucial role on engine performance and emissions. In the present work, a pilot fuel is used to ignite a gaseous methane-air mixture. Three diesel-like pilot fuels are used comparing especially the cetane number (CN) and the aromatic content (AC). The experiments are conducted in a single-cylinder heavy-duty research engine keeping the total-fuel energy constant. Lean conditions are applied for the port-fuel injected methane-air mixture (∅CH4 = 0.52). The methane provides 97% of the total energy while 3% of the energy comes from a pilot fuel. The experiments are performed for two pilot-fuel injection pressures and two engine speeds. The results of the present work suggest that DF combustion consists of three overlapping combustion stages: (I) ignition of the pilot fuel, (II) burning of the main fuel in the vicinity of the pilot fuel, and (III) combustion of the remaining main fuel. It was observed that cetane number directly affects the peak heat-release rate (HRRpeak) during Stage I, whereas aromatic content influences HRRpeak during Stages II and III. A fuel with high cetane number and aromatic content provides high DF combustion efficiency, low methane slip, THC and CO emissions at the expense of high NOx emissions. An increase in the aromatic content is responsible for the increased NOx emissions. Based on the average performance trends of the pilot fuels, they can be rated as [high CN, high AC] > [Low CN, high AC] > [High CN, AC free].