Today, emission legislations for engine exhaust particles are mass based. The engines of today are low-emitting with respect to particle mass, with the emissions approaching the detection limit of the current measurement method. This calls for new and improved measurement methods. Both from the point of view of the engine developers and regarding human health effects, particle number seem to be the particle property of greatest interest to legislate upon. Recently, a proposal for a new particle number based measurement methodology has been put forward by the United Nations Economic Commission for Europe (UN ECE). The gas and particle mixture (the aerosol) of engine exhaust is not a stable system. The size and the number of the particles change over time as the temperature and pressure change. Particle number measurements call for dilution which changes the gas-phase concentrations of the condensing gases. The dilution process alters the conditions in the aerosol and thereby influences the measurements. Within the current project it was desired to better understand the outcome of particle number measurements and the complexities of particle sampling, dilution and conditioning prior to measurements. Two experimental set-ups have been developed within the project. The first system includes a rotating disc diluter followed by a volatility Tandem Differential Mobility Analyser (v-TDMA). The second set-up, called the EMIR-system, includes ejector diluters in series followed by a stand-alone Condensation Particle Counter (CPC). After the development of these experimental set-ups, the v-TDMA has been used to study the volatility and the size distributed number concentration of exhaust particles. The EMIR-system was used for total number concentration measurements including only the solid fraction of the aerosol. The experimental work has given practical experience that can be used to estimate the benefits and disadvantages of upcoming measuring methodology. For the engine developers, in order to produce engines that meet future legislation limits, it is essential to know how the measurement procedure influences the aerosol. In summary, the experimental studies have shown that the number of nucleation mode particles is strongly affected by varied dilution. No upper threshold value of the dilution has been found where the dilution effect diminishes. The volatility studies have shown that it is mainly the nucleation mode particles that are affected by heat. The v-TDMA instrument have shown to be a sensitive analytical tool which, if desired to use for further engine exhaust particle characterization, needs some development work. Experimental work with the EMIR-system, which in principle is similar to the instruments proposed for a future standard, shows that these types of measurement systems are sensitive to small changes in the detector cut-off. The major outcome of the project lies in the new detailed knowledge about particle number measurements from engines. / QC 20100628