In this account of the inactivation of poliovirus and reovirus by bromine in water, emphasis is placed on 2 major points which have received insufficient attention in previous investigations. In all experiments the species of bromine, compound or ion acting on the virus has been established and its concentration given and in all experiments the nature and degree of virion clumping or aggregation has been determined, in most cases, quantitatively described. A few experiments on ECHO virus are included. Methods and apparatus were devised for following disinfection rates at 1/2 sec intervals during the fast reactions of some of the bromine species with water suspensions of single virus particles. Methods have been devised for preparing and storing the viruses with little or no particle aggregation. In general, Br2 and OBr inactivate poliovirus 3-10 times faster than HOBr in equimolar concentrations. Aggregation among the virions of both polio and reoviruses increases their resistance to inactivation by bromine. Survival may be increased over 100 fold by this means. Reovirus infectivity is destroyed by HOBr about 15 times faster than poliovirus but if the reovirus is aggreated it can survive HOBr treatment that destroys dispersed poliovirus.