The composition and chemical polymorphism of the essential oils of several populations of Thymus mastichina subsp. mastichina, T. mastichina subsp. donyanae and T. albicans, which belong to the Section Mastichina of the genus Thymus, were investigated by GC and GC-MS. All representative population samples of these three taxa were characterized by their high 1,8-cineole content, which is a discriminatory and common feature of the essential oils of the Section Mastichina. Linalool is another major constituent of the essential oils of some populations of T. mastichina subsp. mastichina and T. albicans. The essential oil of T. mastichina subsp. donyanae is also characterized by large amounts of borneol. This feature distinguishes its essential oil from the type subspecies. To study the infraspecific variability of each taxon, the results obtained in the GC analyses of the volatile oil of several individual plants from each population were submitted to a Principal Component Analysis and a Cluster Analysis, which showed that the essential oils of the only population investigated of T. mastichina subsp. donyanae have no chemical polymorphism, whereas the oils of T. mastichina subsp. mastichina and T. albicans have infraspecific variability. Some correlation between the amount of linalool in the essential oil of T. mastichina subsp. mastichina and the Atlantic humidity was established.