Twenty-four-hour PM 2.5 and PM 10 samples were collected simultaneously at a highly trafficked roadside site in Hong Kong every sixth day from October 2004 to September 2005. The mass concentrations of PM 2.5 , PM 10-2.5 (defined as PM 10 −PM 2.5 ), organic carbon (OC), elemental carbon (EC), water-soluble ions, and up to 25 elements were determined. Investigation of the chemical compositions and potential sources revealed distinct differences between PM 2.5 and PM 10-2.5 . The annual average mass concentrations were 55.5±25.5 and 25.9±15.7μg/m 3 for PM 2.5 and PM 10-2.5 , respectively. EC, OM (OM=OC×1.4), and ammonium sulfate comprised over ∼82% of PM 2.5 , accounting for ∼29%, ∼27%, and ∼25%, respectively, of the PM 2.5 mass. Low OC/EC ratios (less than 1) for PM 2.5 suggested that fresh diesel-engine exhaust was a major contributor. Seven sources were resolved for PM 2.5 by positive matrix factorization (PMF) model, including vehicle emissions (∼29%), secondary inorganic aerosols (∼27%), waste incinerator/biomass burning (∼23%), residual oil combustion (∼10%), marine aerosols (∼6%), industrial exhaust (∼4%), and resuspended road dust (∼1%). EC and OM comprised only ∼19% of PM 10-2.5 . The average OC/EC ratio of PM 10-2.5 was 7.8±14.2, suggesting that sources other than vehicular exhaust were important contributors. The sources for PM 10-2.5 determined by the PMF model included ∼20% traffic-generated resuspension (e.g., tire dust/brake linear/petrol evaporation), ∼17% locally resuspended road dust, ∼17% marine aerosols, ∼12% secondary aerosols/field burning, and ∼11% vehicle emissions.