A novel chlorine dioxide production apparatus, or reactor, is disclosed. The reactor receives reactants from two or more feed pipes under pressure, and the reactants react in a reaction chamber that operates under elevated pressure and within a specified temperature range. A gaseous product, such as chlorine dioxide, is generated and is released directly into a liquid stream for bleaching, disinfection, and other purposes. Also disclosed is a novel system for production of quantities of chlorine compounds from commercial grades of starting materials, and methods of using the reactor in situ for bleaching, and for disinfection, decontamination, and sterilization of flows of water or other liquids. An automated system for multiple points of addition of chlorine dioxide to a flow to be treated is also disclosed.