Investigations of bromination of naphtho[2,3-c][1,2,5]thiadiazole-4,9-dione revealed that the reaction proceeded most efficiently with N-bromosuccinimide in sulfuric acid and depending on the reaction conditions, can give mono-, di-, tri- and tetrabromo derivatives of naphtho[2,3-c][1,2,5]thiadiazole-4,9-dione. A series of the major isomers was isolated, and their structure was proven using heteronuclear multiple bond correlation NMR spectroscopy and high-resolution mass spectrometry.