Number concentration of particles emitted by combustion engines has recently attracted attention, due to the fact that particles of the size range found in tail pipe emissions are suspected of being hazardous to human health. This paper describes the application of an Electrical Low Pressure Impactor (ELPI) to the measurement of number concentrations of diesel exhaust particles. The size distribution of particles as fine as 30 nm is determined using the aerodynamic diameter as the characteristic dimension. Results were obtained on both the engine and chassis dynamometer, in real-time, for steady state and transient tests. Swedish Environmental Class 1 diesel fuel was used, having a sulfur content of less than 10 ppm wt.A scheme for the calculation of particle losses in the sampling system was developed, showing high penetration of particles under the conditions examined. An estimate of particle density was produced by sampling monodisperse fractions of the exhaust aerosol to the ELPI through a Differential Mobility Analyzer (DMA) under steady state conditions. The effective density values as a function of particle diameter were compared with values obtained by direct comparison of ELPI and DMPS measured size distributions. The values of effective density decreased with increasing particle diameter.The size distribution measurement results present an accumulation mode peak in the range of 30 to 100 nm aerodynamic diameter, depending on the engine operation conditions. The shift in particle size could be explained by coagulation effects, and possibly HC adsorption in the dilution tunnel. Tests in various transient and steady state driving cycles showed considerable differences in size distribution.