Because of increasing concern for both microbial control and disinfection by-products (DBPs) formation, water utilities are strictly examining and optimizing disinfection practices. In this study, modeling of trihalomethanes (THMs) formation at processed water of the Kagithane water treatment plant in Istanbul City was conducted. Data for THMs and other water quality and operational parameters were generated through a 12-month sampling program between January and December 2003. A multiple linear regression model was developed to predict THMs concentrations in processed water. Routinely measured parameters including total organic carbon (TOC), pH, temperature, and chlorine dose were used to develop the model for the prediction of THMs. Both pH (r = 0.963) and temperature (r = 0.921) were found to be the parameters of the highest statistical significance as predictors for THMs occurrence. The regression analysis resulted in a model that is directly applicable to the chlorination of raw waters. This indicated that the linear models developed could be used to estimate THMs concentration for different water quality and treatment processes with different operational conditions.