The Water Treatment Plant Model (WTP Model) is a predictive tool for simulating chemistry within a treatment plant, with a focus on disinfection and disinfection byproducts (DBPs). This tool has been developed to support both USEPA's Stage 1 and Stage 2 Disinfectants/Disinfection Byproducts Rule (D/DBPR) regulatory development efforts. This paper presents the results of a calibration and validation analysis of several key algorithms contained in the WTP Model using full-scale water quality and treatment data collected under the Information Collection Rule (ICR) by large surface water treatment utilities. This work represents a significant effort undertaken to use full-scale data to calibrate a DBP model and to characterize its limitations. The calibration effort resulted in improvements to the several algorithms to better match the central tendency of observed data. Furthermore, while the analysis indicates that the WTP Model is a useful tool in predicting distributions of water quality parameters from a population of plants, its accuracy as a site-specific predictive model without site-specific calibration is limited. Includes tables, figures.