This study aimed to evaluate whether preliminary ultrasound-assisted maceration (UAM) has positive effects on quantitative essential oil separation from leaves of thyme (Thymus vulgaris L.). Moreover, the present work compares the chemical, qualitative and quantitative composition of essential oils obtained from thyme through steam distillation preceded by preliminary classical maceration (CM) in water and UAM over periods of 10, 20, 60 and 120 mins. The 20 min UAM significantly affected the increase of distilled oil yield (about 9%) as compared to the control sample (not modified by maceration or sonification). Longer UAM processing did not have considerable effects on the oil yield achieved from the material studied. The content of the main components received from the oil was also examined. Changes in composition of the essential oil obtained from steam distillation after preliminary maceration were within the range of requirements of the European Pharmacopoeia. After 120 min of CM and UAM, a decrease of thymol by respectively 6% and 12%, carvacrol at respectively 9% and 14%, p-cymene at respectively 14% and 17%, and an increase of γ-terpinene concentration at respectively 53% and 35% was noted in the essential oil. Copyright © 2009 John Wiley & Sons, Ltd.