The premixed charge compression ignition (PCCI) combustion represents a possible solution for decreasing the pollution with respect to diesel engine, while maintaining the efficiency rate at values that are comparable, and in some cases higher, than those of a diesel engine. This paper investigates the operation of an optical-access compression ignition engine (bore: 82 mm, stroke: 90 mm) running at PCCI combustion with neat bio-ethanol and European commercial diesel fuel injected in the intake manifold and into the cylinder, respectively. In its original configuration, the engine burned diesel and this case was used as reference of compression ignition combustion. Then, different amounts of bio-ethanol were injected varying the energizing time of the injector set in the intake manifold. This allowed to create PCCI combustion with high levels of pre-combustion mixing, and to ensure low equivalence ratio and low flame temperatures too. Moreover, both the amount and the start of diesel injection was varied to investigate their effects on the several combustion phases. UV-Visible imaging and spectroscopic measurements were performed in the engine and the autoignition of the charge, the combustion process and the chemical species involved were detected and analysed. In particular, optical diagnostics allowed to observe how the mixture burned: no luminous flame emission in the visible range was recorded; while in the ultraviolet wavelength range numerous species, like HCO, HCOH, OH, and CO and others were detected. Varying the in-cylinder premixed ratio the combustion retarded and the rate of heat release passed from single to three-phase premixed combustion, so revealing a phase due to intermediate temperature reactions. The same behavior was observed varying the start of injection and the quantity of diesel that affected the premixed ratio. Spectroscopic measurements revealed that during the intermediate temperature heat release large amount of OH radical governed the start of combustion of the charge. It was also observed that the preignition combustion was mainly due to the stratified mixing of the diesel fuel close to the bowl wall. Finally the presence of OH radical was monitored for the whole combustion process