A new secoiridoid compound was isolated from the leaves of Olea europaea. This compound, not previously identified, is the bis methylacetal of oleuropein aglycone, the 3,4-dihydroxyphenylethyl [(2,6-dimethoxy-3-ethylidene)-tetrahydropyran-4-yl]acetate (3,4-DHPEA-DETA), and was found in different olive cultivar phenolic extracts as one of the major secoiridoid components. This compound was shown to be easily transformed in acidic aqueous media into 3,4-DHPEA-EDA, the major polyphenolic compound found in olive oil, and permitted us to increase the yield of 3,4-DHPEA-EDA isolation from the olive leaf extract. The antiradical activity of this new compound, evaluated by scavenging of 2,2-diphenyl-1-picrylhydrazyl radicals, was much higher than the one found for 3,4-DHPEA-EDA or alpha-tocopherol. Results also call to attention the need for a careful identification of compounds by HPLC-MS, usually performed in acidic conditions.