It is well known that ethanol can be used in spark-ignition (SI) engines as a pure fuel or blended with gasoline. High enthalpy of vaporization of alcohols can affect air-fuel mixture formation prior to ignition and may form thicker liquid films around the intake valves, on the cylinder wall and piston crown. These liquid films can result in mixture non-homogeneities inside the combustion chamber and hence strongly influence the cyclic variability of early combustion stages. Starting from these considerations, the paper reports an experimental study of the initial phases of the combustion process in a single cylinder SI engine fueled with commercial gasoline and anhydrous ethanol, as well as their blend (50%vol alcohol). The engine was optically accessible and equipped with the cylinder head of a commercial power unit for two-wheel applications, with the same geometrical specifications (bore, stroke, compression ratio). Ultra-violet (UV) natural emission spectroscopy measurements ranging from 250nm to 470nm wavelength and simultaneous thermodynamic analysis were used to better understand the effect of ethanol content on flame kernel inception and development. All experiments were conducted at wide open throttle (WOT), with stoichiometric air-fuel mixtures, fixing the engine speed at 2000rpm. Optical investigations allowed to follow the evolution of chemical species that marked the spark discharge (cyano CN and hydroxyl OH radicals) as well as flame front initial growth (OH and carbyne CH radicals). Vibrational and rotational temperatures were calculated during the arc and glow phase by the ratio between the emission intensity of CN and OH radicals. Results were compared with adiabatic flame temperature traces obtained by applying a two zone model.