Dual fuel combustion represents a possible solution for decreasing the emission produced by diesel engine, while maintaining the efficiency rate at comparable values. In this activity, methane and diesel are used for dual fuel combustion. Methane is injected in the intake manifold, when the intake valves are closed; meanwhile the diesel is delivered directly into the cylinder. The aim of this work is to study the effect of increasing methane concentration at two engine speeds (1500 and 2000 rpm, respectively), on the start and evolution of dual fuel combustion in a compression ignition research engine using infrared diagnostics.Infrared diagnostic has the advantage of being not intrusive, it gives a high spatial and temporal resolution of combustion phenomenon and it is able to qualitatively identify the local concentration of chemical species. Infrared camera detects the energy that the target emits as electromagnetic waves from 3 to 5.5 μm wavelength. Hence, no light sources or multiple optical accesses are needed; and commercial fuels can be used because they emit in the infrared as well.In the experiments, two filters are adopted to follow the whole combustion phenomenon from injection of diesel fuel to late combustion phase with high spatial and temporal resolution. Using the band-pass 3.3 μm, qualitative information about fuel vapour distribution and ignition location during low and high temperature heat release has been provided. Moreover, the combustion evolution is investigated by means of the band-pass filter at 4.2 μm, which is the characteristic of the asymmetric stretch of the CO2 molecule. In particular, autoignition and combustion evolution of the DF combustion are investigated increasing premixed ratio of methane. Differences with the conventional diesel combustion are highlighted. Diesel is the driven fuel that manages the autoignition and combustion of the DF mode in the investigated test cases.