A methodological approach to phenolic profiling making extensive use of LC-MS with extracted ion chromatograms was applied to extracts of five different olive tissues: pulp, seed, stone, new-season leaves, and old-season leaves. Tissue extracts of the cultivars Hardy's Mammoth, Corregiola, Verdale, and Manzanillo were analyzed by HPLC with UV and ESI MS detection. Chromatograms of samples of green Hardy's Mammoth drupes, a uniquely Australian olive cultivar, were dominated by a large, broad peak. This peak was not attributable to oleuropein, which is usually the dominant phenolic compound in green olive fruit, but the phenolic compound I. This compound was isolated by semipreparative HPLC and characterized by 1D- and 2D-NMR. Extraction studies showed that the compound was not likely to be an artifact of an enzymatic degradation process. Tritium labeling studies were used to establish a possible relationship between the biosynthesis of I and oleuropein.