This paper presents a brief description of the hydraulic characterization components of the IDDF protocol. The hydraulic characterization component is designed to provide a systematic approach to developing the RTD function for the specific reactor or process train. The data is then used in conjunction with the rest of the IDDF model to determine the overall inactivation of the target microorganism and to develop an overall strategy for optimizing the disinfection process. The hydraulic characterization component of the IDDF protocol consists of 3 levels, which provide a wide range of effort and accuracy for determining the RTD function. Users of this protocol should select the level appropriate for the amount of accuracy desired in the determination of the contactor or process train hydaulic characteristics. Includes 13 references, tables, figures.