This study was designed to examine the chemical composition and in vitro antioxidant activity of the essential oil of Clinopodium vulgare. GC-MS analysis of the oil resulted in the identification of 40 compounds, representing 99.4% of the oil; thymol (38.9%), gamma-ter-pinene (29.6%) and p-cymene (9.1 %) were the main components. The samples were subjected to a screening for their possible antioxidant activity by using 2,2-diphenyl-1-picrylhydrazyl (DPPH) and beta-carotene-linoleic acid assays. In the first case, IC50 value of the C. vulgare essential oil was determined as 63.0 +/- 2.71 mu g/ml. IC50 value of thymol and gamma-terpinene, the major compounds of the oil, was determined as 161 +/- 1.3 mu g/ml and 122 +/- 2.5 mu g/ml, respectively, whereas p-cymene did not show antioxidant activity. In beta-carotene-linoleic acid system, C. vulgare essential oil exhibited 52.3 +/- 1.19% inhibition against linoleic acid oxidation. In both systems, antioxidant capacities of BHT, curcurnine and ascorbic acid were also determined in parallel experiments. (c) 2006 Elsevier Ltd. All rights reserved.