The use of oxygenated and renewable fuels is nowadays a widespread means to reduce regulated pollutant emissions produced by internal combustion engines, as well as to reduce the greenhouse impact of transportation. Besides PM, NOx and HC emissions, also the size distribution of particles emitted at the engine exhaust represent meaningful information, considering its adverse effects on the environment and human health. In this work, the results of a comprehensive investigation on the combustion characteristics and the exhaust emissions of a GDI high performance engine, fuelled with pure bio-ethanol and European gasoline, are shown. The engine is a 4-cylinder, 4-stroke, 1750 cm₃ displacement, and turbocharged. The engine was operated at different speed/load conditions and two fuel injection strategies were investigated: homogeneous charge mode and stratified charge mode. The particle size distribution at the engine exhaust was measured in the range 5-1000 nm using a differential mobility spectrometer. Simultaneously, the air-fuel mixing and the combustion process inside the cylinder were followed by means of high-speed imaging in the UV-visible. To this aim, the last cylinder was optically accessible through a sapphire window (5 mm wide) in the engine head. The particles size distributions measured at the engine exhaust for both gasoline and bio-ethanol show similar trends, suggesting a common route for the particles formation. For bio-ethanol the accumulation mode is shifted towards smaller sizes, which could be ascribed to the chemical and physical properties of the fuel, as observed by optical investigation of the fuel injection and combustion process. On the other hand, an increase of particles number concentration for stratified charge operating mode was observed for both fuels, due to the poorer air fuel mixing and the diffusive combustion of the fuel films deposited on the piston and cylinder wall.