Aerosol measurements were carried out at Kosan, Cheju Island, Korea for the period from July 20 to August 10, 1994. Total suspended particles were collected by high volume samplers and PM 2.5 particles with gaseous volatile species were collected by a filter pack sampler and their ionic composition are analyzed. The average mass concentration of PM 2.5 particles was comparable to that of PM 3 particles collected during March, 1994 at the same site but the average non sea-salt sulfate concentration was higher that that of PM 3 particles, implying the fraction of anthropogenic air apllutants during this period is higher than that during March, 1994. During the measurement period, two distincitive patterns were observed, high concentrations of mass and water soluble ions were observed between July 20 and August 1 while those during after August 2 were low. Back trajectory analysis results show that air masses arriving at Kosan during the earlier period were mainly from Korea and Japan while those during the later period were from the North Pacific Ocean. It is suggested that the particle ion concentrations during the later period are marine background concentrations at Kosan during the summertime.