Urban aerosol samples for PM10 and PM2.5 were collected during summer (August) and winter (December) 2000 in southern Taiwan (Tainan City) to demonstrate the temporal variations of Hg and As in particulate matter (PM). The mean mass concentrations with standard deviations were 80.0±26.8μgm−3 for PM10 and 50.6±16.6μgm−3 for PM2.5. The average PM2.5/PM10 mass ratio for the two periods combined was 63%, indicating that fine particles were a large portion of PM10. Particulate samples of Hg and As were analyzed within 2 days following sampling and weighing, because of the highly volatile nature of PM Hg and As. The average Hg and As values in PM10, PM2.5 and PM2.5–10 in summer were significantly lower than those in winter. PM2.5 Hg constituted 0.34 to 5.8ngm−3 and PM2.5–10 Hg 0.05 to 3.1ngm−3. PM2.5 As constituted 1.09 to 9.51ngm−3 and PM2.5–10 As 0.18 to 4.14ngm−3. In summer and winter PM10, the Hg contents showed regular daily variation, with the higher values at daytime and lower values at nighttime, indicating conversion of gaseous Hg to the particulate phase by reaction with atmospheric oxidants under strong solar radiation during the daytime in both summer and winter. PM As behaved similar to Hg in the summer, but in the winter higher concentrations were observed during the nighttime than during the daytime, implying that the stable temperature inversion during winter nighttime caused the accumulation of PM As near the ground. In summer, SE–WSW winds carried As from an As-emitting fossil power plant to the sample area. In a similar vein, NE–WNW winter winds contributed to aerosol Hg, especially in PM2.5, originating from a waste incinerator located NW of Tainan City.