Although tax regulations provide an incentive for buying a car with reduced C02 emissions, in the coming years the share of diesel vehicles in the business segment is likely to grow, thus unintentionally threatening air quality. This is because the NOx emissions of a modern Euro 5 diesel car are still very high in practice. Despite the NOx Euro standards for diesel vehicles having been substantially tightened since 1992, in practice there proves to have been very little progress, with Euro 5 diesel vehicles still emitting approximately the same amount of NOx as 21 years ago, as the present study shows.