A cinnamic acid ester, two dihydrocinnamic acid esters, a noroxyneolignan, a dibenzyl ether derivative, along with common aromatic compounds and flavonoids were isolated from the extracts of fresh leaves and twigs of Oxalis pes-caprae. The structures of these aromatic compounds were elucidated on the basis of spectroscopic analyses, especially 2D-NMR techniques. Their effects on the germination and growth of Lactuca sativa L. as standard target species have been studied at concentrations ranging from 10(-4) to 10(-7) M. The activities of some compounds were higher than that of pendimethalin, a commercial pre-emergence herbicide.