This paper reports experiments on a single-cylinder direct-injection compression ignition engine operating in premixed charge compression ignition (PCCI) combustion mode. The engine was fuelled with pure rapeseed methyl ester (RME) and bio-ethanol. RME was injected in the combustion chamber by common rail (CR) injection system at 800 bar and bio-ethanol in the intake manifold by commercial port fuel injection system at 3.5 bar. The effects of different percentage of bio-ethanol were studied by means of both the in-cylinder heat release analysis and the high-speed UV-visible chemiluminescence visualization. The pollutant formation and exhaust emissions of the engine operating in dual fuel mode were evaluated.The increase of the bio-ethanol content improved the brake thermal efficiency slightly even if the brake fuel consumption increased. However, the choice to inject two biofuels decreases both the smoke opacity and NOx concentration. These results were analyzed considering the data obtained by spectroscopy measurements in the combustion chamber. In particular, the high presence of radical species such as OH, HCO, and H₂CO contribute to describe the chemical reactions and physical processes involved in dual fuel combustion mode.