Optical diagnostic was applied to undiluted engine exhaust to supply a low cost and real time evaluation of the oil dilution tendency of selected fuels. Specifically, UV-visible-near IR extinction spectroscopy was applied in the exhaust line of a Euro 5 turbocharged, water cooled, DI diesel engine, equipped with a common rail injection system. The engine was fuelled with commercial B5 fuel and a B30 v/v blend of RME and ultra low sulfur diesel. The proposed experimental methodology allowed to identify the contribution to the multi-wavelength extinction of soot, fuel vapor, hydrocarbons and nitrogen oxide. Further, the evolution of each species for different post-injection interval settings was followed.On-line optical results were correlated with off-line liquid fuel absorption values. Moreover, spectroscopic measurements were linked to in-cylinder pressure related data and with HC and smoke exhaust emissions.The late post injection activation and fuel amount increase did not affect the energy release within the cylinder for either fuels. As a consequence, soot and NOx exhaust emissions remained constant during the post-injection setting. The HC exhaust increase was correlated to post-injection amount and followed in the time by UV extinction signal. The blend B30 attained a lower UV signal that was related to higher oil dilution tendency of biofuel.