An examination of stable isotope compositions of precipitation in Northeast Asia from the IAEA/WMO data set reveals that many sites have a distinct winter-summer seasonal pattern in deuterium excess (d-) values that appears to reflect the diverse air masses and moisture sources affecting the regions during the summer and winter seasons. To further understand this pattern, stable isotope data were obtained for individual rainfall events over a two-year period (May 1995-May 1997) at Cheju Island, Korea. The deuterium excess values, which range from 3.0 to 40.6 permil, show a distinct seasonal variation with higher d-values in winter (> ~15 permil) and lower values in summer (~10 permil). Such a seasonal variation appears to be closely related to different air masses affecting the island during different seasons: a cold-dry continental polar air mass in winter and a hot-humid maritime tropical air mass in summer. Cheju Island is strongly affected by the Asian Monsoon. In the winter, a strong continental high pressure system develops and advects continental polar air that has formed over a land surface that is largely covered by snow. Before reaching the study site at Cheju Island, however, these air masses are modified and gain moisture as they pass over the Yellow Sea. Under such environmental conditions, the precipitation tends to be isotopically enriched. During the summer, a thermally driven low-pressure system develops over the Asian continent, leading to cyclonic circulation and a southerly flow over Cheju Island. This flow drives the movement of warm, moist maritime tropical air masses over the island. Moisture during these times tends to be isotopically depleted due to the low evaporation rates in its source region and amount effects due to the nature of summer precipitation mechanisms.