In order to obtain a fundamental understanding of how soot forms and burns up during combustion in diesel engines, the relation between exhaust smoke level and combustion duration in direct injection diesel engine was measured. The effect of different injection timings, engine speeds and loads on combustion duration are presented and discussed. Combustion experiments in the less complex environment of a continuous spray open flame were performed from which soot samples were collected at various locations in the flame. By use of a special technique were obtained information on the mechanism of soot formation and oxidation could not be obtained from engine experiments. The similarity of soot formation and oxidation within diesel combustion chamber and continuous spray open flame are discussed. An analysis of these results shows that the exhaust smoke level is mainly determined by the concentration of soot at the time of exhaust valve opening, i.e. combustion duration is very important. The net soot release in a diesel engine cylinder, for a given fuel, is controlled by the amount of soot oxidation and the rate of combustion.