To measure traffic pollutants with high temporal and spatial resolution under real conditions a mobile laboratory was designed and built in Helsinki Polytechnic in close co-operation with the University of Helsinki. The equipment of the van provides gas phase measurements of CO and NO x , number size distribution measurements of fine and ultrafine particles by an electrical low pressure impactor, an ultrafine condensation particle counter and a scanning mobility particle sizer. Two inlet systems, one above the windshield and the other above the bumper, enable chasing of different type of vehicles. Also, meteorological and geographical parameters are recorded. This paper introduces the construction and technical details of the van, and presents data from the measurements performed during an LIPIKA campaign on the highway in Helsinki. Approximately 90% of the total particle number concentration was due to particles smaller than 50nm on the highway in Helsinki. The peak concentrations exceeded often 200,000 particlescm−3 and reached sometimes a value of 106 cm−3. Typical size distribution of fine particles possessed bimodal structure with the modal mean diameters of 15–20nm and ∼150nm. Atmospheric dispersion of traffic pollutions were measured by moving away from the highway along the wind direction. At a distance of 120–140m from the source the concentrations were diluted to one-tenth from the values at 9m from the source.