Measurements in the exhaust plume of a petrol-driven motor car showed that molecular cluster ions of both signs were present in approximately equal amounts. The emission rate increased sharply with engine speed while the charge symmetry remained unchanged. Measurements at the kerbside of nine motorways and five city roads showed that the mean total cluster ion concentration near city roads (603cm−3) was about one-half of that near motorways (1211cm−3) and about twice as high as that in the urban background (269cm−3). Both positive and negative ion concentrations near a motorway showed a significant linear increase with traffic density (R2=0.3 at p