Concentrated liquid biocide compositions are produced by: A) feeding (a) bromine atoms and chlorine atoms in the form of (i) one or more of BrCl, (ii) Br2, and (iii) Cl2 into (b) water containing sulfamate anions, or feeding each of (a) and (b) into a reaction vessel, such that the numerical ratio of the total number of bromine atoms fed to the total number of chlorine atoms fed is in the range of 0.7:1 to 1.7:1; and B) providing enough alkali metal base in the mixture being formed in A) to keep the pH of such mixture in the range of 7-11 during all or substantially all of time feeding in A) is occurring, the amounts of (a) and (b) used being amounts that form an active-bromine-containing solution in which (i) the active bromine content is 50,000 ppm or more, and (ii) the atom ratio of nitrogen to active bromine originating from (a) and (b) is greater than about 0.93; and wherein if any sulfate is present in the solution as formed, the molar ratio of sulfate to sulfamate in the solution is less than about 0.2.