The Surface Water Treatment Rule (SWTR) specifies C x T values for achieving various degrees of inactivation of pathogenic microorganisms, with C being the concentration of the disinfectant in the effluent from any reactor and T being a characteristic contact time dependent on reactor hydraulics. The disinfection credit calculated for reactors using the SWTR-prescribed C x T approach is compared with predictions based on accepted disinfection kinetics and fundamental reactor behavior. This analysis demonstrates that the C x T approach lacks a sound conceptual basis, tends to be somewhat conservative, and is unnecessarily costly for many water treatment plants. A dimensionless parameter, k CO, the product of a disinfection rate constant, the effluent disinfectant concentration, and the theoretical detention time, provides a systematic approach for evaluating disinfection conditions and plant modifications. The decay of chlorine in the reactor is considered, and an error in a previous Journal AWWA article is corrected. Includes 8 references, tables, figures.