During an extended field campaign at a high-alpine site (Jungfraujoch; 3580 m asl, Switzerland) from March 1997 to May 1998, the particle number size distribution (diameter D = 18-750 nm) and number concentration N (D > 10 nm) were measured on a continuous basis. The number size distribution was dominated by particles with D < 100 nm, while most of the surface area was in the accumulation mode size range (0.1-1 μm). Average size distributions exhibited a distinct bimodal shape which is generated and maintained by cloud processes and is less distinct in the free troposphere. Fitted modal diameters and standard deviations σ of the Aitken (20-100 nm) and accumulation modes were surprisingly constant throughout the year (D_Ait = 43 ± 3 nm; D_Acc = 140 ± 6 nm; σ_Ait = 2.13 ± 0.11; σ_Acc = 1.61 ± 0.03). The relative number concentrations in both modes are responsible for the seasonality observed in the shape of the size distribution. The high seasonality of N_Acc with summer and winter values of ∼260 and ∼40 cm^-3, respectively, is mainly due to transport of planetary boundary layer air to the station. In contrast, new particle formation is responsible for the high concentrations of nucleation mode particles (D < 20 nm) which exhibited a maximum during the winter months. The relatively low seasonality found for N_Ait (summer, winter values: ∼530, ∼310 cm^-3) is due to gas-to-particle conversion as well as transport processes. An analysis showed that a significant fraction of nucleation mode particles were formed by photochemical reactions.