As a part of ABC-EAREX 2005 (Atmospheric Brown Cloud–East Asia Regional Experiment 2005), trace gases and compositions of PM2.5 were measured at Gosan supersite in Jeju Island during 13–30 March 2005. Aerosol constituents were determined continuously on site at 1-a intervals. The mean concentrations of gases were 56 ppbv for O3, 297 ppbv for CO, 3.2 ppbv for SO2, and 2.2 ppbv for NOx. For PM2.5, the average mass concentration was 27.3 μg/m3, and mean concentrations of nss-SO42−, organic carbon (OC), and elemental carbon (EC) were 3.34 μg/m3, 4.22 μg/m3, and 1.00 μg/m3, respectively. The simultaneous measurements of gaseous and aerosol species revealed that the composition of PM2.5 was mainly determined by anthropogenic plumes from nearby lands. Most of pollution plumes were associated with the passage of cold frontal systems, when all major species were greatly enhanced. Of these, two episodes were followed by dust incidents. In general, EC and nss-SO42− were well correlated with CO while OC was in good agreement with O3. Particularly, the variations of OC/EC ratios exhibited a maximum in the afternoon corresponding to the peak of O3/CO ratios, suggesting OC/EC as a marker representing the degree of chemical processing of fine aerosol. The ratios of OC/CO for all measurements fell between emission ratios of China and South Korea. For pollution episodes, the correlations of CO with nss-SO42− and EC were significant and their relative enhancement was suggested as an indicator to distinguish different types of pollution plumes.