This paper describes research and development activities dealing with a technique which allows the measurement of gaseous and particulate concentrations inside the combustion chamber. This so-called fast-timed gas sampling technique was used for both the observation of the development of gaseous pollutants and soot during combustion and expansion and for getting information about the particle size history. The system has been applied to a modern passenger car DI diesel engine (Volkswagen). The investigation covers the early combustion phase beginning with the start of combustion and throughout the expansion phase until exhaust valve opening. Particles with a size of about 10 nm up to 1 μm were found. Slight variations in the smaller size classes could be observed during the combustion and expansion process.