Does ORP behave differently for chlorine vs. ozone? Any specific ORP value describes the oxidation potential of the water, irrespective of the source or nature of the disinfectant in use. Our experience with model systems thus far, however, is that ORP measurement is more straightforward in chlorinated water than ozonated water. Chlorinated water maintains a relatively constant ORP until the "chlorine demand" of suspended organics and inorganics exceeds the capacity to maintain free chlorine in the water. In contrast, in laboratory studies, ozonated water stabilized at 800mV fell rapidly to 250mV following the introduction of bacterial contaminants or organic material to the water. Bacteria were not recoverable (nonviable) within the few seconds necessary to conduct the first sampling. A high oxidation potential from ozone was clearly available in the water and ozone injection continued throughout the study. Surprisingly, the ORP probe could not measure the rate of reaction. As available oxidation was completed the ORP values climbed slowly back to the original 800mV level. Trevor V. Suslow, Ph.D. U.C. Davis