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Nine Tunisian Thymus capitatus (L.) Hoff. et Link. populations harvested in six bioclimatic zones extending from the sub humid (550 mm rainfall/year) to the lower arid (96.3 mm rainfall/year), and grown at altitudes ranging from 60 to 600 m were assessed for their phenolic composition. The identification of phenolic compounds was revealed by UHPLC-DAD-ESI/MS n . 11 phenolic compounds including three phenolic acids (rosmarinic acid, salvianolic acid A, salvianolic acid E), three flavanones (hesperidin, eriodictyol and naringenin), two C-glycoside isomers of the flavone apigenin, one flavanol gallocatechine, one flavanonol taxifolin and one phenolic monoterpen (carvacrol) were identified. Differences in phenolic composition among populations, in relationship with altitudes and/or annual rainfall were shown. Higher amounts of total phenols, total flavonoids, rosmarinic acid and carvacrol were detected in samples from the most arid zones (96.3–167 mm rainfall/year) in comparison with populations located in wet lands (450–660 mm rainfall/year). The altitude within or between contiguous climates is a main factor influencing phenol compounds content. The chemical variation among the populations should lead to the selection of plants with high potential in order to use them in health care and food industry. [ABSTRACT FROM AUTHOR]