The Diesel process was studied and characterized by ultraviolet-visible extinction and scattering spectroscopy. Measurements were performed on an optically accessible Diesel engine, realized by modifying a single-cylinder, air-cooled, 4-stroke Diesel engine, by means of an external combustion chamber with three optical accesses. Two of them were along the longitudinal direction, for the extinction measurements, and the third along the orthogonal direction, for the scattering measurements. The optical measurements were performed with a temporal resolution of 0.05 ms and with a spatial resolution of 0.1 x 1 mm(2). The simultaneous use of broadband extinction and scattering coefficients in the above-mentioned spectral range permitted the real time evaluation of size, concentration of droplets and soot particles and their optical properties. Aromatic hydrocarbons and carbonaceous matter exhibit strong absorption bands in the 200-350nm spectral range. At fixed time or crank angle, the optical properties change with the wavelength, hence each measurement furnishes independent information on fuel droplets and soot particle size inside the combustion chamber. The experimental results were compared with a model simulation based on Mie theory for the fuel droplets and on Rayleigh's theory for soot particles.