The Global Precipitation Chemistry Project collects precipitation by event to determine the composition of precipitation and processes that control it in five remote regions. Ion balances based on major inorganic species revealed consistent anion deficits at certain sites. This and other evidence suggested that weak organic acids contributed to free acidity. Accurate and precise techniques were developed to measure organic anions and total acidity in precipitation. Twelve samples from a remote site were analyzed for major organic and inorganic chemical constituents. Formic and acetic acids were found in all aliquots which had been treated with a biocide. The disappearance of these acids from untreated aliquots corresponded to a proportionate decrease in free acidity. Weak organic acids contributed 64% of free acidity and 63% of total acidity to precipitation during part of the 1981-1982 wet season at Katherine, Australia. Unmeasured proton donors contributed 21% of total acidity during the period.