Twenty-four-hour PM2.5 and PM10 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 PM2.5, PM10-2.5 (defined as PM10 − PM2.5), 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 PM2.5 and PM10-2.5. The annual average mass concentrations were 55.5 ± 25.5 and 25.9 ± 15.7 g/m3 for PM2.5 and PM10-2.5, respectively. EC, OM (OM = OC × 1.4), and ammonium sulfate comprised over ∼82% of PM2.5, accounting for ∼29%, ∼27%, and ∼25%, respectively, of the PM2.5 mass. Low OC/EC ratios (less than 1) for PM2.5 suggested that fresh diesel-engine exhaust was a major contributor. Seven sources were resolved for PM2.5 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 PM10-2.5. The average OC/EC ratio of PM10-2.5 was 7.8 ± 14.2, suggesting that sources other than vehicular exhaust were important contributors. The sources for PM10-2.5 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.