Abstract The essential oils, isolated by hydrodistillation, from the flowers and the remaining aerial parts (leaves plus stems) from three populations of Thymus carnosus Boiss., collected at different harvesting periods and in three collection sites of Portugal (QL, LM and LA) were analyzed by GC and GC/MS. The oil yields from the flowers collected in LA, QL and LM were six, four and one-fold higher, respectively, than the average oil yields from the remaining aerial parts. Monoterpenes were dominant in all oils (>86%), and sesquiterpenes accounted for 1â6% of the total oils. Diterpenes, phenylpropanoids and non-terpenoid compounds were present in all oils (0.1â0.6%, trace amounts and t-0.4%, respectively). Borneol (26â31%) and camphene (9â18%) dominated QL oils and borneol (18â23%), terpinen-4-ol (11â19%) and camphene (8â10%) were the main components of LM oils. LA oils showed high variability in the dominant components, cis-sabinene hydrate (14%), terpinen-4-ol (12%) and borneol (11%) being the main components in the flower oil. Terpinen-4-ol (18%) and borneol (18%) dominated the vegetative phase oil, whereas borneol (21%) and terpinen-4-ol (16%) dominated the aerial parts oils collected in May and July, respectively. QL oils can be classified as a borneol/camphene chemotype and one of the LA samples as borneol/cis-sabinene hydrate/terpinen-4-ol chemotype. However, all the other LM and LA samples do not fi t within the previously defi ned chemotypes.