Approximately ten years ago, modifications were made to the Oakville Water Purification Plant (WPP) in Oakville, Ontario, Canada, to allow the use of chloramines for residual maintenance. Since then the drivers for secondary disinfection in this system have changed. Progress is being made with the ongoing efforts of the Accelerated Replacement Program for the Cast Iron Water Mains in the South Halton System (Cast Iron Main Replacement Program), the benefits of which include improved disinfectant residual persistence and a decrease in colored water incidents in historical problem areas. In an effort to manage growth, the Oakville system (a 109 ML/d conventional plant) was merged with the Burlington system (a 218 ML/d direct filtration plant) in the fall of 1999. Both systems are owned and operated by the Regional Municipality of Halton (Region). Of concern were the potential impacts associated with blending: although both plants treat surface water from Lake Ontario, free chlorine is used for residual maintenance in the Burlington system. After a detailed cost-benefit analysis, the water supplier opted to examine the feasibility of using free chlorine for residual maintenance in both systems, rather than operating the joined systems with a blend of chlorinated and chloraminated water. Following the recommendation to proceed to a full-scale trial with free chlorine in the Oakville system, implementation and contingency plans were prepared. Evaluation criteria and a public notification strategy were developed as part of the implementation and contingency plans. The one year trial was initiated on October 18, 1999 and continued to October 31, 2000. During this period, regulations affecting drinking water supplies in Ontario were changed. This did not affect the goals and objectives of the one year full-scale trail, nor the evaluation criteria established for the trial. However, the introduction of the drinking water regulation affected how the systems were operated and how the data were reviewed. A distinction was made for data collected before August 26, 2000 (and therefore subject to the 1994 Ontario Drinking Water Objectives) and for data collected after August 26, 2000 (and subject to the 2000 Ontario Drinking Water Standards). The results from the one year full-scale trial are presented in this paper. The recommendation to proceed with the one year full-scale trial was made to the Region in June 1999. Implementation and contingency plans, including the identification of evaluation criteria and the development of a public notification strategy, were prepared during the summer of 1999. The full-scale trial commenced on October 18, 1999 and continued through to October 31, 2000. On October 18, 1999, the ammonia feed system at the Oakville WPP was turned off and water carrying a free chlorine residual was distributed to the Oakville system. The goal of the one year full-scale trial with free chlorine in the Oakville system was to assess the performance of the Oakville system when using free chlorine for residual maintenance. To meet this goal, the objectives of the one year full-scale trial were to: demonstrate that the water in both the Oakville and Burlington systems is of similar quality; demonstrate the persistency of a free chlorine residual in the Oakville system and perform a direct comparison of the Oakville system with the Burlington system, where both systems are using free chlorine; and, demonstrate that the water quality in the Oakville system has been maintained or improved, relative to operating the system with chloramines. Includes tables, figures.