There is a lack of knowledge of volatile organic acids (VOAs), other than formic and acetic acids in gas phase, and this is at least partly due to the lack of sensitive enough measurement methods. In the present study we developed an in situ GC–MS measurement method for measuring C2–C7 monocarboxylic VOAs at ambient air concentration levels, which we used to measure ambient air concentrations in a boreal forest site. In addition, found mixing ratios were compared with PTR-TOFMS data.