Particle size distribution and particle number emissions rather than legislated particulate mass emissions from diesel engines are subject of rising concern especially in the US and several Western European countries *. Recently also particle number emissions from gasoline engines attracted much attention since these engines are supposed to emit very high numbers of ultrafine particles or even nano-sized particles . The work described in this paper focused on the impact of modern diesel exhaust gas aftertreatment systems like diesel oxidation catalysts (DOC) and diesel particulate filter (DPF) on particle number emission. Especially the effect that these aftertreatment systems are supposed to significantly increase ultrafine particle numbers (because they may act like “reactors” actively producing ultrafine solid particles) gave reason for investigating the effects and mechanisms more in detail.Within the work described it became evident that measuring conditions play a decisive role in terms of ultrafine particle emission investigations. Inadequately adjusted dilution parameters like dilution ratio and dilution air temperature can provoke ultrafine particle formation downstream DPF and DOC obviously caused by condensation effects. The investigations also revealed that ultrafine particles formed downstream DPF and DOC do not have a solid carbonaceous core which is consistent to the model that these particles are condensation products.The results of the program clearly depict the lack of a global standard for particle size distribution measurements. Mandatory standards for gravimetric particulate mass sampling are not sufficient in terms of ultrafine particle emissions. Yet, to avoid mis-interpretation of particle size distribution measurements a common standard is urgently needed.