abstract four samples, from two populations, of the liverwort marchesinia mackaii were collected on madeira and at the mainland of portugal, at different collection times. the oil yields determined by hydrodistillation attained 0.5% (v/w). the essential oils isolated by distillation-extraction were analyzed by gc and gc/ms. the main components (>5%) were -asarone (23-31%), 2,4,5-trimethoxyallylbenzene (10-23%), (e)-methyl isoeugenol (12-19%), -asarone (11-13%) and caryophyllene (7-16%).