abstract the chemical composition of the essential oils from the leaves and the flowers of thymus mastichina collected at different regions of portugal were analyzed by gc and gc/ms. it revealed that fresh flowers produce larger amounts of oils than the leaves. the highest oil yields were obtained from the flowers collected at arr'abida (3.5%, v/w) and m'ertola (3.0%, v/w). two types of oils were differentiated according to their main components. oils rich in 1,8-cineole whose concentrations ranged from 39.4% (in the flower oils collected at m'ertola) to 69.2% (in the leaf oils from arr'abida), and an oil rich in linalool (73.5%) that was only detected in a sole plant group from sesimbra. camphor, borneol, terpinen-4-ol and -terpineol appeared in relative high levels in the oils of the plants from m'ertola and s. br'as de alportel. in spite of the samples collected at arr'abida belonging to the 1,8-cineole type, camphene, (e)--ocimene, camphor and borneol were the compounds detected in the smallest amounts in contrast to the two remaining oils. besides the linalool contents determining the chemotype of the plants collected at sesimbra, other compounds appeared in different concentrations to the remaining samples making it worthy to differentiate between them. thus, those samples had the lowest concentrations of many monoterpenes and the highest levels of t-cadinol.