Systematic experiments were performed to examine the inhibitory effects of oxygen on perchlorate reduction and to identify the components and branches in the electron transport chains involved in perchlorate reduction and aerobic respiration by Dechlorosoma sp. KJ (ATCC strain BAA-592). To investigate the effect of dissolved oxygen (DO) on perchlorate reduction, anaerobically grown cultures of Dechlorosoma sp. KJ were exposed to near-saturated concentration of DO for various periods of time. It was determined that cells exposed to oxygen for more than 12 hours were incapable of reducing perchlorate. It was determined that cells exposed to oxygen for more than 12 hours were incapable of reducing perchlorate. Cells exposed to oxygen for less than 12 hours, when re-introduced to anoxic conditions, quickly reduced the redox potential to highly negative values (-127 mV to -337 mV) and were able to reduce perchlorate or chlorite.