The Wet-Weather Flow Technologies Pilot of the Environmental Technology Verification (ETV) Program, which is supported by the U.S. Environmental Protection Agency and facilitated by NSF International, has recently evaluated the performance of chemical induction mixers used for disinfection of wet-weather flows. The MASTRRR Company and USFilter tested a series of chemical induction mixers (5-, 10- and 20-horsepower) at the hydraulic laboratory facilities of United States Geological Survey’s Conte Anadromous Fish Research Center, Turners Falls, MA. Each induction mixer was tested separately in a rectangular flume, incorporating a channel section 7 feet wide with a water depth of 7 feet at nominal flow velocities of 0.5 feet/second, 1.25 feet/second, and 3.0 feet/second. Rhodamine WT tracer dye was used as a surrogate for the chemical disinfectant. A sampling rig was positioned at locations 5 feet, 10 feet, and 15 feet downstream of the mixer to collect dye samples over the entire cross section of the flume to characterize the size and nature of the “chemical” plume. The mixers produced a roughly circular plume with higher concentrations in the center. Smaller plume areas were observed under higher flow velocity conditions; as the energy imparted by the mixer became smaller in relation to the kinetic energy of the flowing water in the flume, the level of mixing observed also lessened. At the lowest flume velocity (0.5 feet/second), the tracer concentrations were more evenly distributed across the flume cross-section. These data suggest that a mixer sizing criteria of between 0.46 and 0.53 horsepower per million gallons per day is appropriate for mixing applications with open channel flow velocities between 0.5 and 3.0 feet/second. It is clear that flow velocity significantly influences the ability of the mixers to disperse tracer effectively and therefore flow velocities must be considered when selecting the appropriate size chemical induction mixer.