Essential Oil Content and Constituents of Thymus × citriodorus L. at Different Phenological Stages R. Omidbaigi * 1 , F. Fattahi 1 and A. Alirezalu 1 1 Department of Horticulture, College of Agriculture, Tarbiat Modares, University, P. O. Box 16415-381, Tehran-Iran Abstract: Twenty- two constituents were found in the essential oils extracted at before flowering, beginning of flowering and at full flowering stages, but twenty-three components in the essential oil extracted from the aerial parts at fruit set stage was identified. Among them geraniol was the major (54.2 - 72.5 %) component. The highest geraniol (72.5 %) was identified from the essential oil at before flowering stage and the lowest amount (54.2 %) from the essential oil of herb of fruit set stage was measured. Geranial content increased from before flowering to fruit set stage. The lowest geranial content (3.2 %) was identified at before flowering and the highest amount (11.9 %) obtained from the essential oil of fruit set stage. Key word index: Lemon thyme, Thymus × citriodorus L., Lamiaceae, essential oil constituents, geraniol, geranial. Introduction: Lemon thyme (Thymus × citriodorus L.) is a perennial sub shrub medicinal plant belonging to the Lamiaceae (Labiatae) family. It is native to southern Europe and is cultivated in the Mediterranean region 1,2 . Probably it is a hybrid between Thymus vulgaris L. and Thymus pulegioides 3,4 L. with loose, upright stems and branches from its base, it is 20 - 40 cm tall. The shoots and leaves are densely covered with fine hairs. The leaves are opposite, gray - green with white borders. The inflorescence tiny and pink color 2 . The whole plant has a pleasant, lemon scent. It is cultivated as a culinary herb, which is used in tea mixture. It has also been used as diaphoretic and for bronchitis. The oil has been found to possess antimicrobial and antifungal activities 4,5 . Stahl- Biskup and Holthuijzen 4 , reported that lemon thyme contains an essential oil rich in geraniol (up to 60 %). Other compounds identified include geranyl acetate (1.0 %), geranyl butyrate (0.8 %), nerol (2.8 %) and citronellol (0.3 %). They mentioned that the lemon fragrance is due to geranial and neral amounting to 8.2 and 5.5 % respectively. Thymol content was also found in small (0.5 %) yet significant amounts 4 . ISSN 0972-060X *Corresponding author (R. Omidbaigi) E- mail: < romidbaigi@yahoo.com > Jeobp 12 (3) 2009 pp 333 - 337 333 Received 09 February 2008; accepted in revised form 17 December 2008