A sesquiterpenic keto-alcohol, cyperolone, has been isolated from nut-grass (Cyperus rotundus (Cyperaceae)) and shown to have the stereo-structure I (R=H) of a novel skeleton based on the following evidence. The spectral data showed the presence of a secondary hydroxyl, an acetyl, an isopropenyl, and a tertiary methyl group. Cyperolone gave the saturated dihydro-derivative (II; R=H). Oxidation of the ketols (I and II; R=H) afforded the cyclopentanones (III and IV), respectively. LiAlH4 reduction of cyperolone formed the diol (VI; R=H) whose NMR [nuclear magnetic resonance] spectrum indicated the acetyl bearing carbon in cyperolone to be quaternary. Alkali treatment of the dione (IV) gave the monoketone (VII) which was synthesized from 14-noreu-desmanone (X). The [beta]-configuration of the C-3 hydroxyl and the C-5 acetyl was deduced from application of the benzoate rule and from a positive Cotton curve of the ketol acetate (VIII; R=COCH3), respectively. The cis relationship of the C-3 and C-5 substituents was confirmed by the presence of an intramolecular hydrogen bond in the diol (VI; R=H).