To better understand the distributions of low molecular weight (LMW) monocarboxylic acids (monoacids) and their sources in the forest, we conducted simultaneous collection of gaseous and particulate samples at a deciduous broadleaf forest site in northern Japan. LMW normal (C 1 –C 10 ), branched chain (iC 4 –iC 6 ), hydroxyl (lactic and glycolic) and aromatic (benzoic) monoacids were detected in the gas and particle phases. The dominant LMW monoacids in gas phase were formic (mean: 953 ng m −3 ) and acetic (528 ng m −3 ) acids. In particle phase, we found that isopentanoic (159 ng m −3 ) and acetic (104 ng m −3 ) acids are dominant species together with lactic acid. Concentrations of LMW monoacids did not correlate with SO 4 2− that was used as an anthropogenic tracer, indicating that LMW monoacids are derived from the local sources within the forest ecosystem. Concentrations of C 1 –C 6 monoacids in gas phase showed positive correlations (r 2 = 0.21–0.91) with isobutyric acid (iC 4 ), which is produced by soil microorganisms. These monoacids are closely linked to the microbial process in soils. Isopentanoic acid in particle phase showed a positive correlation with lactic acid (r 2 = 0.98), which is produced by soil microbes. The observed high abundances of isopentanoic acid are involved with soil microbial activity. We found that acetic acid in particle phase positively correlated with nonanoic acid (C 9 ) (r 2 = 0.63), suggesting that formation of acetic and nonanoic acids are associated with the oxidation of unsaturated fatty acids. We found that forest floor with soil microbes contributes to the emissions of gaseous and particulate LMW monoacids. Our results suggest that forest ecosystem is an important source of organic gases and aerosols in the atmosphere.