Euonyminol [octahydro-2,5 a-bis(hydroxy methyl)-2,9-dimethyl-2H-3,9a-methano1-benzoxepin-4,5,6,7,8,9,10-heptol] is an alkaline hydrolysis product of the alkaloids isolated from Tripterygium wilfordii Hook (Celastraceae). In the parent alkaloids the polyhydroxy nucleus has eight esterified hydroxyl groups, one free hydroxyl group and an O atom involved in a cyclic ether linkage. Euonyminol crystallizes in the monoclinic space group C2 with cell dimensions a = 16.323 (7), b = 8.314 (7), c = 11.809 (5) A and fl -96.7 (1) °. Data were collected on an automatic diffractometer and the structure was solved by direct methods. The 25atom model was refined to an R index of 0.056. The structure consists of two trans-fused six-membered rings and a five-membered cyclic ether formed by fusion of two axial substituents. Euonyminol is related to the polyhydroxy nuclei of other Celastraceae alkaloids such as maytoline and evonine, and a best molecular fit program has been used to compare the structures.