In this paper the emissions of regulated and unregulated exhaust gas components of a fleet of diesel passenger cars measured at Volkswagen in the eighties are compared with the results of a new investigation on modern direct-injection diesel vehicles. The potential of improved diesel fuels to reduce emissions is also examined.The emissions of regulated exhaust gas components as well as fuel consumption have been reduced significantly in the last years as a result of the systematic further development of conventional swirl chamber engines and exhaust gas after-treatment as well as the introduction of SDI/TDI engines. As was to be expected, this has also had a positive effect on the emissions of unregulated exhaust gas components. It has been possible, for example, to reduce the polycyclic aromatic hydrocarbons adsorbed on diesel particulates by more than 95%.The following assessment criteria were used for the comparison of the emissions using different diesel fuels: maximum ozone forming potential, polycyclic aromatic hydrocarbons (PAH) and PAH nitro derivatives, air toxic components and particulate number and size distribution.The emissions resulting from the use of the improved diesel fuel were distinctly better in all assessment criteria. It can be concluded from the results of the investigations carried out that the use of such diesel fuels should be promoted for environmental reasons.