Major accuracy for prediction tools like CFD codes require precise experimental validation. The ultraviolet and visible light absorption and scattering (UV-VIS LAS) is proposed for characterizing air-fuel mixture formation. UV-VIS LAS technique is employed to quantitatively determine spatial concentration distribution of vapor fuel, in combination with simultaneous liquid length and spray penetration measurements by means of Mie-scattering and Schlieren. Decane, Hexadecane and a 50/50 of both fuels have been chosen for this study, to evaluate mixing formation under Diesel conditions. Work has been performed at an optical engine under non-reacting atmosphere, with ambient pressures and temperatures up to 7.3 MPa and 900 K.Fuel optical properties for the two paraffines under engine conditions have been analyzed, and fuel concentration distribution has been obtained for pure fuels. For each of them, mass fraction has been measured, combining attenuation signal with an adiabatic mixing model. Spray characterization shows that evaporation is different between fuels, which suggests that vapor fuel distribution along the spray axis should be different. UV-LAS shows vapor fuel mass distribution along spray axis. For different engine conditions, vapor fuel mass fraction measurements are in agreement with spray characterization results.