A new leucoanthocyanidin, isomelacacidin, has been found with its 4-epimer, melacacidin, in the heartwood of three species of Acacia. Extraction and separation of the epimers is described, and melacacidin obtained pure and crystalline. Isomelacacidin behaves as a reactive p-hydroxybenzyl alcohol and readily forms an ethyl ether by reaction with ethanol, and this distinction from melacacidin facilitates its separation therefrom. The comparative inertness of melacacidin is attributed to an unexpected conformational stability in its 2(eq),3(ax),4(eq)-half-chair conformation, which inhibits resonance stabilisation of the 4-carbonium ion and so reduces the benzylic character of the 4-hydroxyl group.