Abstract Links between phenology, yield and composition of the essential oil of common thyme, Thymus vulgaris L., grown in Guadalajara (Central Spain) were determined in the different phases of the biological cycle during one year. Data showed an average yield of about 2%. The analysis of the oil components was carried out by GC-FID and GC/MS. The main oil constituents were thymol (36.3â47.5%), p-cymene (13.0â27.8%) and Î³-terpinene (5.3â16.2%), which is in accord with a thymol chemotype. The highest yield of oil was obtained in the period of full flowering and the highest concentration of thymol in the period of initial flowering.