Measurements are reported of particle number concentration at two urban sites within Birmingham, UK. The measurements were made with a condensation particle counter (TSI Model 3022A) and an ultrafine particle counter (TSI Model 3025) operated in tandem. Other colocated measurements included particle surface area by epiphaniometer, sulfur dioxide, oxides of nitrogen and ozone, solar radiation or JO1D, and in some campaigns, particle number size distribution by Scanning Mobility Particle Sizer. Events were identified when substantial excursions in particle number count were accompanied by a divergence between the two particle counters indicating a large number of particles in the 3--7 nm diameter range. Criteria have been developed to distinguish between particles within this size range arising from homogeneous nucleation within the atmosphere and from emissions from point sources. Out of a total of 232 days sampling distributed throughout the year, events on only 8 days could be attributed fairly unequivocally to homogeneous nucleation processes, with a further 4 days possibly attributable. Nucleation events occur in winter as well as summer months, but depend, among other things, upon a low particle surface area. Modeling of aerosol dynamical processes for 20 June 1999, indicates that the observations can be explained by a nucleation rate of 6 ¿ 106 cm-3 s-1 at a condensable vapor concentration (as H2SO4) of 6.5 ¿ 107 cm-3. It is inferred that both the nucleation and particle growth processes involve condensable molecules other than, or in addition to, sulfuric acid.