Liquid chromatography/ion trap mass spectrometry with an atmospheric pressure chemical ionization (APCI) interface in the negative-ion mode, using collision-induced dissociation (CID) of precursor ions in parallel with UV-diode-array detection (DAD), was applied to the characterization in crude extracts of the water-soluble antioxidant phenolics from Maté leaves (Ilex paraguayensis). APCI gives unequivocal information about both the molecular mass of these compounds and some indications about their structure, which were confirmed by the UV-DAD fingerprints and by the diagnostic CID patterns. This allowed the identification of 10 constituents, including the 3 naturally occurring isomers of caffeoylquinic acid (CGA), namely neo-chlorogenic acid, chlorogenic acid and crypto-chlorogenic acid, as well as 3 isomeric dicaffeoyl quinic acids, rutin (quercetin-3-rutinoside), a diglycosyl derivative of luteolin, and 2 isomeric caffeoyl-glucosides. The aqueous extract of Maté, quantitatively analyzed for polyphenol and CGA content (HPLC), was tested for antioxidant activity in both a cell-free system (quenching of the stable free radical DPPH, 2,2-diphenyl-1-picryl-hydrazyl) and in a model membrane system (phosphatidylcholine liposomes). The results demonstrate that the extract is a strong radical scavenger, considerably more potent than the prototype chlorogenic acid, and this indicates a cooperative antioxidant interaction among its polyphenol components. © 1998 John Wiley & Sons, Ltd.