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The absolute configuration of the predominant Cecropia hormone, methyl 12,14-dihomojuvenate, has been determined to be methyl (E,E)-(lORllS)-(+ )-10,11-epoxy-7-ethyl -3,11-dimethyl-2,6-tridecadienoate (I). The less abundant hormone, methyl 12-homojuvenate, can be presumed by analogy to have the corresponding 3,7,11-trimethyldienoate structure (II). The assignment has been established with microamounts of substance by applying Horeau's method to the glycol derivative (III) of the hormone.The course of the perchloric acid-catalyzed epoxide ring opening of I was checked by conducting the conversion in 180-labeled water. It has been ascertained that the configuration at the secondary hydroxyl group of the resulting III remained unchanged. On the other hand, the hydration proceeded with a surprisingly high rate of cis opening.The relative configurations of the two Cecropia juvenile hormones (I, II) have been known for some time (1, 2). The two double bonds in the molecules have the trans or E configuration, whereas the principal carbon chain is cis oriented at the oxirane. This implies R configuration on one of the carbon atoms of the epoxide and S configuration on the other. Recently we reported optical rotatory dispersion data for a mixture of the two natural hormones, and thereby demonstrated [M] yield §