Continuous daily measurements of airborne particles were conducted during specific periods at an under-ground platform within the subway system of the city center of Stockholm, Sweden. Main emphasis was placed on number concentration, particle size distribution, soot content (analyzed as elemental and black carbon) and surface area concentration. Conventional measurements of mass concentrations were conducted in parallel as well as analysis of particle morphology, bulk-and surface composition. In addition, the presence of volatile and semi volatile organic compounds within freshly collected particle fractions of PM 10 and PM 2.5 were investigated and grouped according to functional groups. Similar periodic measurements were con-ducted at street level for comparison. The investigation clearly demonstrates a large dominance in number concentration of airborne nano-sized particles compared to coarse particles in the subway. Out of a mean particle number concentration of 12 000 particles/cm 3 (7500 to 20 000 particles/cm 3), only 190 particles/cm 3 were larger than 250 nm. Soot particles from diesel exhaust, and metal-containing particles, primarily iron, were observed in the subway aerosol. Unique measurements on freshly collected subway particle size fractions of PM 10 and PM 2.5 identi-fied several volatile and semi-volatile organic compounds, the presence of carcinogenic aromatic compounds and traces of flame retardants. This interdisciplinary and multi-analytical investigation aims to provide an improved understanding of reported adverse health effects induced by subway aerosols.