A method of producing a stream of aqueous chlorine dioxide sterilizing fluid suitable for use in sterilizing medical and dental equipment or for wound irrigation or skin asepsis is described. Steps are: pumping fluid portions of a first reagent and a second reagent into the proximal end of an elongate reaction chamber (24) to form a reaction mixture, the reagents being selected to react together to form aqueous chlorine dioxide; providing a stream of water around or adjacent to the distal end of the reaction chamber (24); continuing to pump fluid portions of said reagents so as to cause the reagent mixture to travel through the reaction chamber and then into the stream of water to form a stream of aqueous chlorine dioxide sterilizing fluid; wherein the pumping rates and the internal dimensions of the elongate reaction chamber (24) are selected so that the reaction to form chlorine dioxide is substantially complete when the reagent mixture exits the reaction chamber (24); and continuing pumping fluid portions of said reagents until a desired quantity of said sterilizing fluid has been produced; wherein the pumping of each reagent is carried out by means of a diaphragm pump (90, 92) or a piston pump via a pressure-control valve (32, 34) so that each fluid portion is pumped at substantially constant pressure.