Factors that could affect microbial fuel cells (MFC) operation: the type of proton exchange and the effect of oxygen contamination on MFC operation were examined. A MFC containing membrane was inoculated with G. metallireducens using growth medium containing ferric citrate and bacteria, but power generation was initially very low (0.1 mw/sq m). When L-cysteine was added to the anode chamber at 334 hr with fresh medium, the power output increased to 7.8 mw/sq m within 451 hr, generating power that was an order of magnitude larger than that achieved with the salt bridge MFC using the same bacterium and L-cysteine. The increase in power generation following the addition of L-cysteine confirmed that dissolved oxygen was contributing to the original low level of power generation. Power generation was further increased by adding additional acetate at a final concentration of 20 mM at 900 hr. In the salt bridge MFC, only a small potential (< 5 mv) developed across the MFC circuit over the first 100 hr of operation. Low power generation in the salt bridge MFC was primarily due to differences in internal resistance between the two systems. The internal resistance of the salt bridge MFC was 14 times higher than that obtained from the Nafion membrane MFC. This is an abstract of a paper presented at the 228th ACS National Meeting (Philadelphia, PA 8/22-26/2004).