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Field surveys on atmospheric deposition and stream water chemistry were conducted in an evergreen forest in northeastern Thailand characterized by a tropical savanna climate with distinct dry and wet seasons. Atmospheric deposition of ion constituents by throughfall and stemflow was shown to increase in the beginning and end of the wet season, reflecting the precipitation pattern. The pH and electrical conductivity of stream water increased with alkalinity and base cation concentrations due to mineralization of organic matter by the first rain and retention of anions in soil during the start of the wet season. After initial alkalinization, the pH and alkalinity declined rapidly with the highest SO 4 2− concentration displayed in the middle towards the end of the wet season. The magnitude of peaks in SO 4 2− concentration (13.5-60.6 μmol c /L) reflects deposition during the first 2 months of the wet season (March and April) in respective years (60.8-170 mol c /ha). Release of SO 4 2− with H + , which is retained in soil during the early wet season, may cause acidification later in the season. The deposition and concentration of SO 4 2− declined over 6 years. However, the pH of stream water declined with increasing concentrations of SO 4 2− and other major ions. The release of materials accumulated in the ecosystem was facilitated by the decrease in SO 4 2− concentration/deposition and increased precipitation in the middle-late wet season. The retention-release cycle of SO 4 2− largely contributed to both seasonal and interannual variations in stream water chemistry in the tropical savanna climate studied.