Data on mass concentrations of PM10 and PM2.5, obtained at four urban sites in western Taiwan from 1998 to 2000, were analyzed to examine the pattern of seasonal and yearly variations in the PM2.5/PM10-2.5 ratio, the relationship among particle fractions and the variability of each particle fraction. The results were compared with those reported in the literature for urban and non-urban areas in several countries. Even though the annual mean of the PM2.5/PM10-2.5 ratio at a site might fall within a relatively narrow range over several years, the seasonal mean of the ratio could still vary considerably within a year. These results imply that there is no long-term characteristic value of the ratio for a community. Furthermore, different urban areas did not necessarily have similar ranges of the ratio. Results for the relationship among particle fractions and the variability of each particle fraction also indicated significant differences between communities. For the areas where both PM2.5 and PM10-2.5 are moderately correlated with PM10, separate measurements of PM2.5 and PM10-2.5 are needed for a better assessment of the underlying causes for the health effects of particulate matter.