A major pilot plant study currently underway at three Ontario utilities is examining the feasibility of various alternative processes and oxidants for disinfection, organics removal and byproduct control. Each pilot plant treats one of three major surface types: a soft, colored river water, a moderately hard, agriculturally impacted river, and a Great Lakes supply. Pilot process trains and individual unit processes were identical to all three pilot plant locations. The study therefore provided a unique opportunity to compare the response of these waters to similar treatment processes. This paper addresses specifically the comparison of prechlorination to post-sedimentation ozone as it relates to chlorination byproduct and nonpurgeable organic carbon (NPOC) control measures. Chlorination byproducts discussed in this paper include: simulated distribution system (SDS) and instantaneous total trihalomethanes (TTHM), haloacetonitriles (HAN) and haloketones (HK).