The formation and oxidation of soot, light and heavy hydrocarbons, CO, CO2 and NOx in a D.I. diesel engine have been studied by means of direct fast sampling and chemical analysis of the combustion products collected during the combustion cycle. Particular attention has been paid to the histories of each fuel hydrocarbon class analyzing the chemical transformations that the paraffins, and monoaromatic and polyaromatic compounds, contained in a diesel fuel oil, undergo during the combustion cycle. This approach is able to give information on the origin of soot and heavy hydrocarbon emission from a diesel engine.The concentration of the heavy hydrocarbons decreases during the early stages of the combustion cycle and their profile corresponds roughly to the fuel disappearance rate because of the chemical similarity with the fuel compounds. Paraffinic and aromatic fuel compounds undergo a rapid destruction at the time of injection of the fuel and, concurrent, the formation of oxygenated compounds has been found. The concentration of the total PAH decreases during the early stages of the combustion cycle and a preferential conversion of light alkylated PAH has been found.Maximum soot formation occurs during the diffusion burning phase in as little as 10 crank angles reaching locally the concentration of 1.3 mg/Nl, after that it decreases slowly in 40 crank angles up to 0.05 mg/Nl in the exhaust.In the last part of the combustion cycle an increase of heavy hydrocarbon concentration and the similarity with the original fuel indicates that a part of the fuel injected later and quenched on the chamber walls survives oxidation and contributes massively to the final heavy hydrocarbon emission.