Protosappanin A (I), C15H12O5, mp 250-251°C, was isolated from the heart-wood of Caesalpinia sappan L. (appan Lignum). It yielded a triacetate (II) on acetylation, and an alcohol (III) on reduction with sodium borohybride. On alkali fusion, I afforded sappanin (IV) along with small amounts of catechol and resorcinol. On the basis of chemical and spectroscopic evidence, the structure of I was established as 3, 10, 11-trihydroxy-7, 8-dihydro-6H-dibenz[b, d]oxocin-7-one. The result of an X-ray crystal structure analysis supported this conclusion. Protosappanin A (I) seems to be the precursor of sappanin, 2, 3', 4, 4'-tetrahydroxybiphenyl, which has been known as an artificial component of Sappan Lignum since 1872, and may be a metabolite of sappanchalcone. A possible biogenetic relationship of sappanchalcone, brazilin, and protosappanin A is discussed. Protosappanin A (I) has a weak sedative effect in mice.