Acid treatment of an iridoid glycoside results in the cleavage of the acetal bond between the sugar unit and the monoterpenoid aglycon. Iridoids possessing non-conjugated enol ether systems, however, undergo the hydration of the iridoid enol ether functionality in acid medium, as well as the hydrolysis of the bond. We examined the acidrearrangement of secoiridoids such as oleuropein (1) and secologanin