Analysis of a purified fraction from acetone extracts of olive pulp and pomace by electrospray ionisation tandem mass spectrometry (ESI-MSn) showed the presence of oleuropein oligomers, whose occurrence has not been reported previously in the literature. The main ionic species (m/z 1613) in the ESI-MS spectrum was an oleuropein trimer containing three linkages through the hydroxytyrosol backbone. In both samples, oleuropein dimers (m/z 1075), trimers comprising two hydroxytyrosol linkages (m/z 1615), tetramers (m/z 2153) and pentamers (m/z 2691) were also detected by MS. The occurrence of oleuropein oligomers was also observed by nuclear magnetic resonance (NMR). 13C, 13C distortionless enhancement by polarisation transfer DEPT 90, 13C DEPT 135, gHSQC (heteronuclear single quantum coherence) and gHMBC (heteronuclear multiple bond coherence) spectra showed all carbon and proton resonances of oleuropein with the exception of the low-mobility and asymmetric signals of the aromatic rings. Since mature olives were used in this study, it is possible that the disappearance of oleuropein that has been described to occur with the olive fruit maturation, could be associated with the formation of phenolic oligomers together with lower-molecular-weight compounds resulting from its degradation. Copyright © 2006 Society of Chemical Industry