Methyl azulene-1-carboxylate (2) and 1-acetylazulene (3) were treated with sodium monobromoisocyanurate (SMBI) (1) to give 3-bromo-substituted azulenes 5 and 6. respectively. The brominations in dichloromethane gave the brominated products in low yields, while the reactions in dichloromethane-water gave the products in high yields. This indicates that SMBI (1) was hydrolyzed in the presence of water to generate hypobromic acid, which accelerated the bromination as a source of bromonium cation. The protic ethanol also promoted the reactions.