The reaction between bromite and thiosulfate ions in aqueous solution with and without phenol has been studied under batch and now conditions. The bromite oxidation of thiosulfate proceeds via two consecutive fast reactions, the first accompanied by a significant increase, the second by a large decrease in pH. The H+ production is autocatalytic and strongly influenced by phenol. The bromite-thiosulfate-phenol flow system exhibits bistability and large-amplitude pH oscillations. Several relevant reactions such as bromite-phenol, bromite-bromide, bromate-thiosulfate, and bromate-phenol were also investigated, and their participation in the batch and flow behavior of the bromite-thiosulfate-phenol system is discussed. A qualitative explanation of the pH oscillation is proposed.