Great attention was paid to nitrogen oxides and particulate matter emitted by the compression ignition engines. The adoption of methane dual fuelled with diesel could contribute to the reduction of these pollutants. This paper aims to investigate the combustion phenomena occurring when a premixed methane/air charge is ignited by the direct injection of diesel fuel. The research activity was performed on a production compression ignition engine, three-cylinder, 1.0 L, equipped with a common rail injection system. In order to operate in diesel/methane dual fuel mode, the intake manifold of the engine was modified to set an electronic port fuel injector suitable for gaseous fuels. Different engine speeds and loads were tested. For each engine condition, a small part of the total energy was provided by the direct injected diesel fuel while the remaining by the methane. Thermodynamics analysis of the combustion process was performed through conventional measurement involving in-cylinder pressure acquisition. Endoscope based optical techniques were carried out for the combustion visualization with high spatial distribution and temporal evolution. Two-colour pyrometry method was applied to the flame images to evaluate the temperature and the soot concentration. This method allows to achieve a better insight about the pollutant formation. Experimental results revealed that DF combustion occurs with lower temperature and soot formation than diesel operation thus leading to lower nitrogen oxides and particle emissions at exhaust. (C) 2017 Elsevier B.V. All rights reserved.