abstract the effects of selenium treatments [0 (control), 2, 4, 6 and 8 mg 1-1] on essential oil extracted from salvia officinalis l. plants were investigated by gc and gc/ms during two successive seasons 2008 and 2009. essential oil content (% and g plant -1) increased with various treatments of selenium compared with control. the highest essential oil percentage (0.5%) was recorded at 8 mg 1-1 selenium treatment. greatest essential oil yields (0.1 g plant-1) were recorded at 6 and 8 mg l-1 selenium treatments in both seasons. twenty-two constituents were identified in essential oil extracted from salvia officinalis l. aerial parts, accounting for 98.6-99.9% of total constituents, and belong to eight chemical main classes. ketones class was the major one. the major constituents of salvia officinalis l. essential oil were -, -thujone and camphor which increased as selenium level increase. the highest amounts of major compounds [-thujone (55.0%), - thujone (12.7%) and camphor (31.8%)] resulted from 8 mg l-1 selenium treatment. the chemical classes of essential oil such as monoterpene hydrocarbons, oxygenated monoterpenes, sesquiterpene hydrocarbons and oxygenated sesquiterpenes decreased in all selenium treatments compared with control. however ketones class increased as selenium level increase. on the other hand, ethers, aldehydes and esters decreased at all selenium treatments except for 2 mg l-1 selenium treatment were increased compared with control.