Abstract The composition of 12 essential oil samples, obtained by steam distillation of twigs of rosemary (Rosmarinus officinalis L.) harvested in four different locations from southern Spain and during three phenological stages, has been analyzed by capillary GC and GC/MS in combination with retention indices. Quantitative data relative to 53 constituents (accounting for 96.7-99.2% of the oils), 12 of which being reported for the first time in Spanish rosemary oil, are described. All samples studied belong to the chemotype α-pinene/l,8-cineole/eamphor, the main and characteristic components being camphor (17.2-34.7%), α-pinene (10.2-21.6%), 1,8-cineole (12.1-14.4%), camphene (5.2-8.6%), borneol (3.2-7.7%), β-pinene (2.3-7.5%), verbenone (2.2-5.8%), β-caryophyllene (1.8-5.1%), limonene (2.0-3.8%), α-terpineol (1.2-2.5%), myrcene (0.9-4.5%), p-cymene (0.2-3.4%), bornyl acetate (0.2-2.3%), linalool (0.3-1.0%) and terpinen-4-ol (0.4-0.9%). The effect of harvest time on the oil production and chemical composition was examined. The highest oil yields (1.6-1.8%) were recorded during the fruiting period (summer). In general, minimum amounts of camphor and maximum amounts of α-pinene were observed in winter. The concentration of 1,8-cineole was almost constant throughout the year, though other oil constituent levels varied randomly with the plant life cycle. Thus, seasonal and geographical variations on the content of representative components allowed to achieve a quality assessment of the rosemary oils and, consequently, to deduce the best period for processing rosemary in the studied area.