Operability and operation scenario of hydrogen Isotope Separation System (ISS) for ITER and a demo reactor are checked and examined based on the information in the ITER documents. It has been confirmed that the current ISS design well covers the operational requirements of the designated operation modes and other anticipated operations. However, some operation modes using rich deuterium fuel will require additional tritium inventory in the system (not only for ISS but the storage system in some cases) and make duration time between the operations longer. Operation scenario of ISS for a demo reactor will be quite simple, but the process capacity required being associated with detritiation of coolant water become excessively large without any countermeasure to control/reduce tritium permeation in the bleeding blanket operated at a temperature much higher than that of ITER.