Abstract Essential oils of hairy roots and plant roots of Arnica montana L. were obtained by hydrodistillation with the yield of 0.60% and 0.40% of dry weight, respectively. Their constituents were identified using GC, GC/MS, 1H- and 13C-NMR. Fifty-six compounds were identified in both essential oils that consisted mainly of bifunctional thymol derivatives and sesquiterpene hydrocarbons. 10-Isobutyryloxy-8,9-didehydro-thymol isobutyrate and 10-isobutyryloxy-8,9-didehydro thymol methyl ether were the main components of both essential oils.