Formic and acetic acids are ubiquitous trace gases in ambient air, but their sources remain to be fully understood. They originate from photochemical oxidation of volatile organic compounds (VOC). Direct emissions of these carboxylic acids are supposedly negligible at North European conditions. Radiocarbon analysis of formic and acetic acids provides information on the origin of the precursor VOC, since biogenic VOC have the same 14C/12C ratio as atmospheric carbon dioxide, while VOC emitted from fossil fuels are devoid of 14C. The origin of atmospheric formic and acetic acids was studied by accelerator mass spectrometry of air and rainwater samples. Sampling sites were selected at different distances from centers of anthropogenic activities in Europe, ranging from urban via rural to remote sites. The procedure for preparation of samples was checked by several quality assurance samples, and no significant contamination from atmospheric carbon dioxide was observed. The results show very high (55--100%) biogenic contributions to both carboxylic acids in all samples. Only in semiurban and urban areas were the biogenic fractions less than 80%. The results indicate that biogenic VOC can have substantial influence on formation of photochemical oxidation products and thus photochemical oxidants. ¿ 2001 American Geophysical Union