Abstract The essential oil obtained by hydrodistillation of the stems and leaves of Xanthium strumarium L. and the aerial parts of Centaurea solstitialis L. (two Compositae species of Iran) were analyzed by GC and GC/MS. Twenty-two compounds representing 86.4% of the stem oil of Xanthium strumarium were identified, among which bornyl acetate (19.5%), limonene (15.0%) and Î²-selinene (10.1%) were the major ones. The leaf oil of the plant was characterized by higher amount of limonene (24.7%) and borneol (10.6%) among the 28 components comprising 85.2% of the total oil detected. Eighteen components were identified in the oil of Centaurea solstitialis representing 86.6% of the total oil with hexadecanoic acid (30.8%) and caryophyllene oxide (25.2%) as major constituents.