Because of increasing concern about balancing health risks for microbiological control and disinfection byproduct formation, utilities are closely examining and optimizing disinfection practices. The authors present a methodology for developing site-specific, in-plant (finished water) chlorine (Cl2) residual and trihalomethane (THM) formation models. In a case study, the methodology was applied at three operating water treatment plants in the Paris suburbs. A key obstacle was the limited historical record of bromide (Br-) occurrence. However, lab chlorination experiments indicated that approximately 10 percent of Br- was typically incorporated into THMs. In-plant Cl2 residuals were accurately simulated with a simple first-order Cl2 consumption model. The most accurate THM simulations were obtained using a recently developed US Environmental Protection Agency model that incorporates species-specific reactivity parameters. Includes 12 references, tables, figures.