Technical Abstract: Thymus hyemalis L. and Spanish Thymus vulgaris L. shrubs are characterized by a great chemical intraspecific variability among plants. This fact makes it difficult to detect real changes happening in their essential oil composition during the vegetative cycle. Based on this, in the present work, clones of T. hyemalis and T. vulgaris were used to monitor real seasonal variations in the aromatic composition of the essential oil. Shrubs were harvested at five different phenological stages during the plant vegetative cycle. Capillary GC/MS with olfactometry analysis was used to determine the volatile profile of the essential oil samples. GC-Olfactometry analysis was conducted on each phenological stage for T. hyemalis and T. vulgaris. A total of 96 (hyemalis) and 52 (vulgaris) aromas were detected with the 27 (hyemalis) and 26 (vulgaris) most important listed here. The compounds with the most aroma impact for T. hyemalis are linalool, borneol, thymol, and ß-damascenone. The compounds with the most aroma impact for T. vulgaris are eucalyptol, borneol, terpinyl acetate, and ß-damascenone. ß-Damascenone, ß-ionone, and myrtenol are reported for the fist time.