A formulation of triiodide quaternary ammonium strong base resin has been employed on the Space Shuttle in a device known as the microbial check valve (MCV) to impart an iodine residual to water collected from fuel cells in order to prevent subsequent microbial contamination. The next higher homologue in the iodinated resin series is the pentaiodide resin. In practice when all of the quaternary ammonium sites have been loaded with triiodide ions additional diatomic iodine molecules can be added to give pentaiodide resins from 0 to 100% saturation. These resins differ in their aqueous iodine residual as well as biocidal capability. A typical formulation in which 97% of the quaternary ammonium sites are loaded with triiodide and 70% of those are additionally loaded with l2 has been compared with a typical triiodide with respect to biocidal capability.This pentaiodide resin has the capacity to devitalize 1 x 109 bacteria per ml in aqueous suspension within 10 sec of contact with the resin bed. Organisms that have been tested include Legionella pneumophila, Escherichia coli, and Staphylococcus aureus. Similar reductions in plaque forming units have been noted with MS 2 and f2 bacteriophage. The resin is also active against cysts of the intestinal parasite Giardia lamblia after a holding period of three minutes at 37°C. The iodine residual of the 97%/70% pentaiodide resin is 2.0 mg per liter. This residual can be substantially reduced by use of granulated activated carbon or additional ionic exchange resins downstream from the pentaiodide resin. In summary, the pentaiodide resin with 70% of its weight composed of elemental iodine, has a relatively low iodine residual, and may offer superior disinfection capability for applications on long duration space vehicles.