Application of microbial disinfectants is necessary to achieve primary and secondary disinfection. Unfortunately, most disinfectants react with constituents in drinking water to form unwanted disinfection byproducts (DBPs). This is forcing many utilities to optimize their chlorine doses or use chloramines in the distribution system. In addition, some plants may need to optimize existing treatment processes or add advanced treatment technologies to control DBPs. These approaches would be facilitated if chlorine and chloramine decay could be more easily predicted under new dose and tretment conditions. This study developed some practical models that predict chlorine and chloramine decay at different doses or after different treatment processes based on a single disinfectant decay study and results of a simple demand test at the new conditions. Three source waters with varying water quality were collected from three different drinking water treatment plants. Upon arrival total organic carbon (TOC), dissolved organic carbon (DOC), UVA254, pH alkalinity, hardness, turbidity, and ammonia and bromide concentration were measured for each water. A portion of the source water was filtered with a 1-micron filter and set aside for raw water disinfection. The rest of the water was treated in a batch-mixing tank by enhanced coagulation, tapered flocculation and settling, and then filtered with a 1-micron filter. A portion of this water was set aside as finished enhanced coagulated (EC) water. The rest was further treated with bench-scale advanced treatment processes: GAC adsorption (GAC), ozonation followed by biofiltration (O₃/BF), or alone (O₃ Only), and UV irradiation. Following the designated treatment water quality analysis data was collected. TOC and UVA₂₅₄ were measured for each treated water. In addition, DOC, pH alkalinity, turbidity, and ammonia concentration were measured for each EC water. Includes 7 references, tables, figures.