A mass transfer-based model is developed for predicting chlorine decay in drinking water distribution networks. The model considers first order reactions of chlorine to occur both in the bulk flow and at the pipe wall. The overall rate of the wall reaction is a function of the rate of mass transfer of chlorine to the wall and is therefore dependent on pipe geometry and flow regime. The model can thus explain field observations that show higher chlorine decay rates associated with smaller pipe sizes and higher flow velocities. It has been incorporated into a computer program call EPANET that can perform dynamic water quality simulations on complex pipe networks. The model is applied to chlorine measurements taken at nine locations over a 53 hour period from a portion of the South Central Connecticut Regional Water Authority's service area. Good agreement with observed chlorine levels over a range of assumed wall decay constants is obtained at locations where the hydraulics are well-characterized. The model should prove to be a valuable tool for managing chlorine disinfection practices in drinking water distribution systems.