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A structure (I: R1 = R2 = H) is suggested for cassminic acid, a minor constituent from the bark of Erythrophleurn guineense.THE relationship of cassminic acid (I; R1 = R2 = H) C2,H3,,0,) to cassanic acid is shown by a sequence of standard reactions:The presence of a hydroxy-group intramolecularly hydrogen bonded is indicated by i.r. absorption at 3500 cm.-l, unchanged on dilution. Oxidation of the ester (I; R1 = Me, R2 = H) with chromic anhydride or (more specifically) with bismuth trioxide 2 yields a diosphenol (11; R1 = Me or the alternative 5,6-unsaturated structure) (ferric reaction) which shows pH-sensitive absorption at 283 nm. (E 4900). This band moves to 337 nm. (E 2400) in alkali, a shift reversed on reacidification. A diosphenol structure is thus indicated and is confirmed by formation of an enol acetate in the U.V. spectrum of which the main band has moved to 246 nm. and is no longer pH-sensitive. Cassminic acid itself thus contains