This study was part of a joint research project between the University of Houston (UH) and Montgomery Watson Harza (MWH). The ultimate goal of the project was to develop a combined ion-exchange and biological treatment process for perchlorate removal from drinking water. The discussion in this paper focuses on perchlorate reduction in ion-exchange spent brine. The objectives were to: investigate perchlorate and nitrate reduction in actual ion-exchange spent brine; and, to determine the required conditions to stabilize rapid microbial perchlorate reduction in ion-exchange spent brine. Actual ion-exchange spent brine (6% NaCl) was collected from the MWH pilot plant located in La Puente in southern California. The spent brine was kept in the dark, at 4 degrees C prior to use. The brine was treated by sparging with oxygen-free nitrogen gas for 1 hour as well as an addition of 1.5 times the stoichiometric acetate requirement and adjustment of the pH to ~7. A culture developed in previous research and maintained by feeding it perchlorate in a synthetic medium with 3% NaCl was used as inoculum for this research. The culture was fed spent brine diluted to 3% NaCl as a sequencing batch reactor (SBR): after the perchlorate and nitrate were reduced, the culture was allowed to settle for 1-2 hours, 40- 60% of the supernatant was withdrawn and replaced with 3% diluted spent brine. Includes 7 references, table, figures.