Disclosed is a method for controlling microbial growth in an aqueous system containing sulfite and/or bisulfite residues by addition of a peroxy compound at a pH of greater than 5. Also disclosed is a method for stabilizing an active halogen biocide in an aqueous system containing peroxide residues by addition of an N-hydrogen compound to the active halogen biocide before combining it with the peroxide containing aqueous system. Further disclosed is an optimized papermaking biocide program consisting of initially treating sulfite bleached pulp with peroxide followed by application of an N-hydrogen-stabilized active halogen compound to the paper producing white waters and an analytical method for determining peroxide concentrations in aqueous systems in the presence of sulfite and/or bisulfite.