At the Laboratory for Internal Combustion Engines (LVV) of the Swiss Federal Institute of Technology (ETH), soot formation and oxidation during the highly turbulent, unsteady combustion in a DI diesel engine has been investigated. In particular, the aim was to examine the influence of fluid dynamic processes (injection pressure) and thermodynamic parameters (phasing of the start of injection, EGR rate) and chemical kinetics (O2 content). To achieve this aim, a multi-colour measurement system was developed, implemented and successfully used on a single-cylinder research engine equipped with a common rail injection system. At the same time, we developed a new phenomenological soot mode, which was validated by means of measurements using the multi-colour sensor (average temperature, cylinder pressure, characteristic mixing times, etc.). Having been tested at more than 70 different operating points, the new model shows a good agreement with the measurements.