Abstract: Apparatus and continuous-flow process for producing a solution of chlorine dioxide by reaction between a hypochlorite, a chlorite and an acid in a gas generator-reactor comprising a plurality of separate reaction chambers interconnected in series arrangement, in which liquid reactants flow through successive stages, but chlorine dioxide is prevented from flowing between stages. A water-driven venturi eductor removes gaseous reaction products and maintains a vacuum within the reaction chambers. Gaseous reaction products are mixed with the eductor exhaust water. A separate eductor may be provided to remove liquid products of reaction from the gas generator-reactor.