The model discussed in this paper describes the decay of free chlorine and chloramine based on water quality parameters as well as chlorine dosage and chlorine to ammonia ratio. This model is being developed for incorporation into an existing model that predicts concentrations of trihalomethanes (THMs) produced in water treatment processes. This paper describes the experiments performed to generate data for the modeling effort (including utility selection, experimental methods, and results), the modeling approach and resulting model, and an assessment of the model's prediction accuracy. The authors compare the model developed in this project with other applicable models for chlorine and chloramine residual decay. Free chlorine residual decays very rapidly when the ratio of applied chlorine dosage to the TOC of the water is 0.5. The faster decay of free chlorine observed during the first five hours is described better by a second order rate equation than first order rate equation. Two sequential first order rate equations were found to be better than a single first order rate equation to describe the chloramine decay.