An analysis of in-cylinder gas density and oxygen mass concentration (YO2) impact on the mixing and oxidation processes and the final soot emissions in conventional high temperature diffusive Diesel combustion conditions is presented in this paper. Parametrical tests were performed on a single cylinder heavy duty research engine. The density was modified adjusting the boost pressure following two approaches, maintaining the YO2 either before or after the combustion process. The YO2 was modified by diluting fresh air with exhaust gas maintaining a constant density. The possibility of controlling the soot emissions combining both parameters (YO2 and density) is evaluated and, in a final part, the NOx emission results are also addressed. Results show that YO2 has a strong effect on both mixing and oxidation processes while density affects principally the mixing process. Both parameters affect the final soot emissions. The density modification through adjustment of boost pressure modifies the trapped mass and has a strong impact on the evolution of YO2 (thus on the evolution of the mixing process) during combustion. If the density is increased maintaining constant the YO2 at the beginning of the combustion, the NOx-Soot trade-off is enhanced. (C) 2011 Elsevier Ltd. All rights reserved.