A chlorine dioxide solution generating apparatus is provided for use primarily in water treatment. The apparatus has a reaction chamber supplied with metered quantities of an acidic solution and a chlorite or chlorate solution whereby a reservoir of a chlorine dioxide solution is generated and retained within the reaction chamber. A conduit is provided along which a pressurized flow of a motive fluid is fed and with which the interior of the reaction chamber communicates via a dip tube. A venturi is located in the conduit adjacent an outlet from the dip tube whereby suction is applied to the reaction chamber by the pressurized flow of motive fluid. The suction draws chlorine dioxide solution from the reservoir whereby it is entrained into the flow of motive fluid. A controller is provided to control flow of motive fluid and metering of the supply of the solutions to the reaction chamber.
