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A summary is made of studies about acetate and formate in precipitation across the world. Some details of sample preparation and experimental methods used in the analysis of organic acids in rainwater are given. The spatialtemporal distributions of acetic and formic acids are summarized. The observed levels of these acids, together with their theoretical contributions at different levels of pH value, indicate that these acids are important contributors to the acidity of precipitation. The sources of the two organic acids, which can be categorized as natural, anthropogenic, and chemical sources, and their meaning to the other organic acid researches are discussed.