Single bubble cavitation offers an unique opportunity to evaluate the effect of bubble activity in promoting chemical reactions. In this paper we study the isomerization reaction of maleic acid into fumaric acid using an aqueous solution of maleic acid saturated with CH2Br2. The Br* radicals are generated at the bubble surface and a whitish thread forms. For comparison, the same reaction was conducted in a sonochemical bath. A possible scheme of the reactions activated at bubble surface after the decomposition of organic brominated substances is proposed.