Five species of Sonchus subgenus Sonchus were surveyed for their phenolic constituents. Seven flavonoids were identified: luteolin, luteolin-7-glucoside, apigenin, apigenin-3-glucoside, quercetin, quercetin-3-glucoside and quercetin-3-galactoside; three were phenolic acids: caffeic, chlorogenic and isochlorogenic; and two were coumarins: aesculetin and cichoriin. Their systematic significance for this genus is discussed.