The compound perbromic acid, HBrO4, is an oxygen acid of bromine. Other names include hydroxy-γ7-bromanetrione and hydroxidotrioxidobromine. Perbromic acid is unstable and cannot be formed by displacement of chlorine from perchlorate, as periodates are prepared. The acid was discovered when a sample of radioactive selenate, SeO42−, was left to decay. It is a strong acid, and strongly oxidizing, and is the least stable of the halogen(VII) acids. It decomposes rapidly on standing to bromic acid and oxygen, with the evolution of brown bromine vapour.