Artemisinin (ART), or qinghaosu, a sesquiterpene lactone containing an endoperoxide bridge, is an efficient drug against chloroquine-resistant strains ofPlasmodium falciparum in the treatment of cerebral malaria. ART and its derivatives are rapidly metabolized into dihydroartemisinin (DHA), which is responsible for the antimalarial activity. After a simple liquid-liquid extraction (LLE), determination of DHA in plasma was conducted by liquid chromatography-mass spectrometry (LC-MS) with an electrospray ionization (ESI) interface. Addition of dodecylamine to the mobile phase and analysis in selected-ion monitoring (SIM) mode resulted in enhanced sensitivity and selectivity. Use of a primary amine, e.g. propylamine, pentylamine, hexylamine or dodecylamine, in the mobile phase can inhibit the formation of multimer clusters and ion adducts by forming one dominating species (analyte-primary amine adduct). Dodecylamine (DDA) was selected after several assays as a competitive agent for adduct formation. The procedure was validated and precision and accuracy were found to be good for the matrix studied. The method was applied to samples provided by patients receiving artesunate (ARS) for antimalarial treatment.