Isocadiolides A-H (<b>1</b>-<b>8</b>) and cadiolide N (<b>9</b>), new polybrominated aromatic compounds, were isolated from a Korean <i>Synoicum</i> sp. ascidian. On the basis of the results of extensive spectroscopic analyses, these compounds possessed tris-bromohydroxyphenyl moieties as a common structural motif, while their cores varied [cyclopentenedione (<b>1</b>-<b>5</b>), dihydrofuran (<b>6</b> and <b>7</b>), pyranone (<b>8</b>), and furanone (<b>9</b>)], reflecting different extents of rearrangement and oxidation. Several of these compounds exhibited weak antibacterial activities and moderate abilities to inhibit the microbial enzymes sortase A and isocitrate lyase.