Whilst limited information on particle size distributions and number concentrations in cities is available, very few data on the very smallest of particles, nanoparticles, have been recorded. Measurements in this study show that road traffic and stationary combustion sources generate a significant number of nanoparticles of diameter <10nm. Measurements at the roadside (4m from the kerb) and downwind from the traffic (more than 25m from the kerb) show that nanoparticles (<10nm diameter) accounted for more than 36–44% of the total particle number concentrations. Measurements designed to sample the plume of individual vehicles showed that both a diesel- and a petrol-fuelled vehicle generated nanoparticles (<10nm diameter). The fraction of nanoparticles was even greater in a plume 350m downwind of a stationary combustion source. On a few occasions, a temporal association between nanoparticles in the size range 3–7nm and solar radiation was observed in urban background air at times when no other local sources were influential, which suggests that homogeneous nucleation can also be an important source of particles in the urban atmosphere.