2-Hydroxyxanthone, 1,7-dihydroxyxanthone, 1-hydroxy-7-methoxyxanthone, 2-hydroxy-1-methoxyxanthone, 1,2-methylenedioxyxanthone and 2,8-dihydroxy-1-methoxyxanthone were isolated from the wood of Kielmeyera excelsa Camb. The wood of Kielmeyera excelsa Camb. was found to contain 2-hydroxyxanthone, 1,7-dihydroxyxanthone (euxanthone), 1-hydroxy-7-methoxyxanthone, 2-hydroxy-1-methoxyxanthone and 2,8-dihydroxy-1-methoxyxanthone. All these compounds had already been isolated previously from other species of the same genus and were identified by direct comparison with authentic samples. Only one constituent proved to be a new natural compound. The molecular weight, determined by mass spectrometry, fitted a methylenedioxyxanthone structure. Besides the molecular ion peak, this spectrum contained only one more intense peak at M-1 mass units. The ease with which the molecule loses a hydrogen atom was also indicative of a methylenedioxy-benzene system which aromatizes upon electron impact. Of the three possible methylenedioxyxanthones, only the 1,2-derivative was given consideration, in view of its probable biosynthetic relationship to 2-hydroxy-1-methoxyxanthone. Indeed, treatment of the natural isolate with PCl5 and hydrolysis of the reaction product yielded 1,2-dihydroxyxanthone. 1,2-Methylenedioxyxanthone, which has been synthesized, is the fourth natural methylenedioxyxanthone to be reported. Other representatives of this relatively small group include 4-hydroxy-2,3-methylenedioxyxanthone, 4-methoxy-2,3-methylenedioxyxanthone and 1,2,4-trimethoxy-6,7-methylenedioxyxanthone. 1,2-Oxygenation of a xanthone ring is not common among Kielmeyera constituents. Up to now, the only principal metabolites to be characterized by this structural feature were found to occur in K. petiolaris (Spr.) Mart. Within the Guttiferae family, K. excelsa and K. petiolaris form a pair of vicarious species, stemming from a common ancestor, but thriving presently in contiguous, but different ecological environments.