PM2.5 aerosols were collected from an urban and a coastal site in southern Taiwan in January (winter) and April (spring) of 2002. Under controlled relative humidity (RH) of 35±1%, , and were found to be the dominant water-soluble ionic species in both winter and spring. At the urban site, these ionic species accounted for an average 44.6% of PM2.5 mass in winter and 36.0% in spring, while at the coastal site they accounted for 44.7% of PM2.5 mass in winter and 38% in spring, thus demonstrating that secondary aerosols contributed a larger portion of the PM2.5 mass in winter at both sites. In winter, constituted a lower percentage of the PM2.5 mass than and at both sites, whereas in spring were the most dominant chemical species. At constant temperature (25±1 °C) and RH (60±1%), the water mass as a percentage of PM2.5 mass was higher in the coastal area than in the urban area. This was true both in winter (coastal 28.9% versus urban 22.4% in the daytime and coastal 33.0% versus urban 27.4% at night) and spring (32.7% versus 28.4% in the daytime and 35.7% versus 32.7% at night). Nighttime aerosols always had higher water content than their daytime equivalents. The water content as measured at 60±1% RH was noticeably higher than and different from values calculated using the water descending model.