Abstract: Method for preparing biocidal halogenated polystyrene hydantoins. The biocidal polymers poly-1,3-dichloro-5-methyl-5-(4?-vinylphenyl)hydantoin, poly-1,3-dibromo-5-methyl-5-(4?-vinylphenyl)hydantoin, and their monohalogenated alkali metal salts and protonated derivatives have been prepared as porous beads by use of highly crosslinked polystyrene beads as starting materials. The porous beads will be useful in water and air disinfection applications when employed in cartridge filters and carafes (for water), as well as for controlling noxious odor when mixed with absorbent materials in items such as disposable diapers, incontinence pads, bandages, sanitary napkins, pantiliners, mattress covers, shoe inserts, sponges, animal litter, carpets, fabrics, and air filters or the like.