The Surface Water Treatment Rule (SWTR) contains tables of disinfectant CT (concentration x time) values that have been demonstrated to achieve the required degree of inactivation of viruses and Giardia cysts. If the CT product for a water treatment plant meets or exceeds those in the SWTR tables, it is assumed this inactivation has been achieved. The objective of the study was to compare two methods for calculating C and T. Tracer tests were conducted to characterize the behavior of flow through the process units of water treatment plants. Effective CT values, which were calculated based on the residence time distribution and the kinetics of chlorine decay, were compared with the most conservative CT values recommended in the US Environmental Protection Agency guidance manual. A total of 16 tests were performed at three water treatment plants; both step-input and pulse-input tests were performed. Fluoride was uses as the tracer in most of the tests. Includes 7 references, tables, figures.