Five polybrominated oxydiphenol derivatives have been isolated from various Great Barrier Reef collections and one Fijian collection of the sponge Dysidea herbacea: 3,4',5,6,6'-hexabromo-2, 2'-oxydiphenol ( 11), 3,4',5,6,6'-pentabromo-2,2'-oxydiphenol ( 12), 3.4',5,6,6'-pentabromo-2,2'-oxydiphenol 1-methyl ether ( 13), 3,4,4',5,6'-pentabromo-2,2'-oxydiphenol 1,1'-dimethyl ether ( 14) and 3,4',5,6'-tetrabromo-2,2'-oxydiphenol 1'-methyl ether (15). The structure of the first member of this series is determined by a new method involving 13C spin-lattice relaxation data. The contributions of nearby hydrogens to quaternary carbon spin-lattice relaxation times are calculated for various possible structures and compared with the experimental data, leading to an unequivocal proof of structure. The structures of the remaining compounds in the series are established principally by analysis of 13C chemical shifts and 13C- 1H coupling constants.