PM2.5 and PM2.5-10 samples were collected between August 1998 and March 1999 at Taichung urban and coastal sites. The major objective in this study was to identify the source contributions to PM10 in different air quality conditions. Principal component analysis with varimax rotation and a chemical mass balance model were used to qualify and quantify, respectively, the source contributions to PM10. Vehicle emissions was the most important source of PM10 at the urban site, followed by crustal materials, secondary aerosols, biomass burning, industrial emissions and marine spray. There was a similar pattern of sources at the coastal site, except that marine spray was found more significantly than the urban site. Although biomass burning and secondary aerosols were not main sources during clean air quality periods, they were the influential sources causing the increase of PM10 to "episodic" levels at both sites.