In the course of tritium handling using a certain tritium processing system, the tritium concentration at the outlet of the system changes with time in a manner peculiar to the system when a gas stream containing tritium is introduced because tritium is apt to be trapped on the surfaces of the system. This phenomenon is called the system effect. A study on the behavior of tritium at the outlet of a processing system could lead to erroneous results if the system effect is neglected. A way to quantify the system effects of a processing system is discussed. The system effects are classified into static system effect and kinetic system effect. The former represents the total amount of tritium to be trapped on the tritium facing surfaces of the system and the latter represents the synthetic result of kinetic behavior of tritium in the subsystems that compose the whole system. The system effect of the experimental piping system is well expressed by applying the serial reactor model to the piping system when the isotope exchange reaction between tritiated water in the process gas and water on the surface of piping materials is dominant. Accordingly, it is concluded that the application of the serial reactor model makes it possible to evaluate the system effects when the dominant reactions in each subsystem of the system are specified.