Abstract: Chlorine dioxide generation systems and methods are disclosed. One or more reactants, such as sodium hypochlorite, may be electrolytically generated on-site for delivery to a reaction column. Low concentration reactants may be used to generate chlorine dioxide as part of a mixed oxy-chloro product stream containing free available chlorine. In at least one embodiment, an optical analyzer may be positioned along a reactant feed line to measure a concentration of reactant supplied to a reaction column. A controller may adjust a flow rate of the reactant in response to information provided by the optical analyzer. The controller may also perform pH control within the system. In some embodiments, the chlorine dioxide generator may be incorporated in an all-liquid water disinfection system.