The aim of present study was to compare the antioxidant potential of two Thymus species on the basis of the chemical compositions of oils obtained by hydrodistillation. In the case of Thymus sipyleus subsp. sipyleus var. sipyleus, 71 compounds were identified representing the 92.5% of the total oil. The major constituents of the oil were described as borneol (11.2%), alpha-muurolol (9.2%), (3-caryophyllene (7.6%), geranial (7.3%) and neral (5.4%). On the other hand, 47 compounds were identified representing 98.7% of the oil of Thymus sipyleus subsp. sipyleus var. rosulans. This oil is characterised by the high monoterpene fraction (94.0%) and especially by the presence of the phenolic carvacrol (58.1%), thymol (20.5%) and their precursors p-cymene (4.1%) and gamma-terpinene (4.4%). The oils were also subjected to screening for their possible antioxidant activity by using 2,2-diphenyl-1-picrylhydrazyl (DPPH) and beta-carotene/linoleic acid assays. In the first case, the free radical scavenging activity of the essential oil of T. sipyleus subsp. sipyleus var. rosulans was superior to var. sipyleus oil (IC50 = 220 +/- 0.5 and 2670 +/- 0.5 mug/ml, respectively). In the case of linoleic acid system, oxidation of linoleic acid was effectively inhibited by T. sipyleus subsp. sipyleus var. rosulans (92.0%), while the var. sipyleus oil had no activity. In the latter case, the linoleic acid inhibition rate of var. rosulans oil is close to the synthetic antioxidant BHT (96.0%). (C) 2004 Elsevier Ltd. All rights reserved.