This paper presents the results of experimental and theoretical investigations on measuring particulate emissions of diesel engines in a dilution tunnel. The results offer a contribution to understanding the influence of several parameters on the particle phase of exhaust gas when diluted and mixed with air. These parameters include the exhaust gas temperature, the dilution ratio of the exhaust gas in the air, the mixture temperature, the flow and mixture conditions, the amount of filter loading and the filter material. In order to determine which physical/chemical processes dominate particle formation in diluted exhaust gas, the results of calculations in terms of condensation and adsorption are compared with the experimental findings. An increase in measured particulate concentrations is generally favoured by short sampling times, fast mixing processes, high exhaust gas temperatures, low mixture temperatures and low dilution ratios. Furthermore, the results show that adsorption of hydrocarbons to soot particles rather than condensation is the major influence on particulate formation.