The acidic volatile components in Midwest Scotch spearmint oil(Mentha gracilisSole) were investigated by means of GC, GC/MS and other analytical techniques. A total of 46 acidic components (35 carboxylic acids and 11 phenols) were identified from the oil. Among them, three carboxylic acids peculiar to spear mint oil, cis-2-pentylcyclopropane-1-carboxylic acid, 3-isopropenylpentane-1,5-dioic acid, and 3-isopropenyl-6-oxoheptanoic acid were newly assigned by spectral data and by synthesis. The absolute configuration of one of the compounds was determined to be (IS, 2R)-(+)-enantiomer by an enantioselective multidimensional GC. The quantitative comparison of the major acidic components in the other three types of spearmint oil, in two types of peppermint oil, and in cornmint oil were discussed.