In order to visually and quantitativelyanalyze mixing behavior within a multichamberozone contactor, this investigation appliedthree-dimensional laser-induced fluorescence(3D LIF) to a model reactor that was scaleddown from a full-scale ozonation plant currentlyin service. Results indicated that mixingconditions inside the contactor were lessthan optimum and were characterized byextensive short circuiting from one chamberto the next, internal recirculation within thechambers, and dead zones with little mixingat the chamber centers.The multichamber ozone contactor is themost widely used design for water disinfection,but its efficacy is diminished by nonideal flowbehaviors. The 3D LIF system described in thisarticle, combined with ozone contactor modelsoftware developed by the authors, provides ahighly effective tool for reactor diagnosis,retrofitting to improve performance, and newreactor design. The information provided canbe used to optimize baffle and chamber configurationsin multichamber ozone contactorsto improve disinfection efficiency and minimizedisinfection byproduct formation.Includes 17 references, figures.